Blake's (Yamaha) take on the YC73 keybed (BHS, balanced hammer standard) is that it is similar to the Montage 8 (classic/1st gen)'s keybed which would be a little different than the MODX8.  The MODX8 uses a graded keybed (GHS, graded hammer standard) which means the keys get progressively heavier from the highest to lowest key. There are plenty of opinions to read.  If you're happy with the YC88 then the YC73 seems to be a great fit in order to lighten the load.  Blake's opinion is that the action has a lighter feel than the natural wood action of the YC88 but everyone (even those who complain about the BHS action) feel that for piano/EP playing the YC73 has a good keybed.  The main complaint I see is that the YC73's BHS keybed is too heavy for the liking of YC73 organ players.  The YC88 would be even a "worse" keybed for organ playing to these critics.  

The Montage 8, to me, is pleasant for piano/EP playing.  The goal of the Montage's keybed is to be enough of a "piano action" and also cover all the synth/organ sounds too.  So it's a bit of a hybrid and, to me, nice and responsive on both fronts.

As far as what you would lose (other than weight) by going to the 73: the YC88 features triple sensor technology.  What this means is that in terms of piano, a triple sensor would simulate escapement of an acoustic piano which allows quickly repeating notes without fully lifting up the key to its resting position before striking a new note.  On a 3 sensor keybed like the YC88 you don't have to lift the key as high in order to repeat the same note (restrike the same key) as you would a 2 sensor mechanism like the YC73 (and Montage 8).  

The CP88 also has a triple sensor and, for piano, that's one of my favorite keybeds for piano.  Granted I've only played for about an hour in the music store.  That said, I think I have some sense of the feel of the YC88.  

So you'll certainly be changing some things going from one of my favorite keybeds to one of my 2nd favorite keybeds.  Your personal sensitivity to the difference is an unknown (to me).  And, in my opinion, these two keybed choices are phenomenal and closely matched.  Overall, given the YC's strength as a modeled organ - I would probably prefer the YC73 given its overall lighter feel vs. the YC88.   For pure organ the YC61 would be the best fit but we don't need to go there since your preference seems to be to optimize for the piano action vs. organ action.  And you can find plenty of folks who weren't happy with the compromises they felt the YC61 keybed made for their own expectations.   A die-hard organ player who rarely uses piano would be a good fit for the YC61.

Others have also suggested getting a YC61 and use the cost savings vs the 73 or 88 to get a MIDI controller that has more keys with a heavier action.  This way you can switch between the 61's native keybed for synth/organ and use the MIDI controller for piano.   Personally, I don't see this as a good fit for either you or myself because I want to carry the least amount possible.  One trip.  One board.  Make it reasonably light.  I think I know where you're coming from on this front.

Sorry - not Bad Mister - and surely don't have the same level of hands-on with these various keybeds.  My hands-on with any of these actual boards is fairly limited.  But I believe Blake when he mentions the YC73 is similar to the Montage 8 - a keybed I've got more familiarity with (although returned the Montage 8 due to weight - still, spent a lot of time on this keybed before and after sale and years past with Motif XF8, etc).

Forgot to add and can't easily edit - so ...

Both the YC88 and YC73 use "standard width" keys.   It's a common complaint that Yamaha's synth action keybeds use a shorter measurement for an octave of keys (skinnier keys, less spacing key to key center-line).  This didn't bother me in the past as I was regularly gigging and practicing with a MO6 keyboard which uses the "skinnier" keys.  Lately I've been doing more of my practice on a "piano" which I find makes it difficult to adjust to the skinnier synth action keys (Montage 7 in my case).  The YC61's span of an octave is shorter but the YC73 and YC88 both use a more standard longer octave span relative to the Montage 7/YC61.  

Therefore, no compromise on that front going from YC88 to YC73. 

I did see Blake's comparison of BHS to the BHE keys of the Montage 8 in a video, but I think more than anything else that might have been a reaction to it being a hammer action keyboard where the lowest keys feel no heavier than the lightest. Other than that particular characteristic, I think the actual BHS (Balanced Hammer Standard) action is, probably much closer to the (Graded Hammer Standard), as their names imply (i.e. graded and balanced versions of the same basic design). And in fact, if you check spare parts (i.e. syntaur.com), you'll see that the actual black and white keys for the CP73/YC73 are the identical part as the top range of keys of the assorted GHS models (and not the same as any BHE models). They just maintain that lightness all the way down, instead of being graded to heavier weightedness on lower keys.

So to the OP, if you like the feel of the MODX8, the YC73 should not feel too different from the highest range of keys of that keyboard. That said, even GHS can feel different from one keyboard to another, for unclear reasons. Maybe because it is mounted differently in different chassis, maybe the presence of speakers in some models affects the subjective experience of playing the board's keys, maybe it's the programming of how the sounds responds to the action in a particular board, maybe some combination of these and possibly other things, but people do report liking GHS better in some models than others. That said, MODX8 seems to be a model where I've seen more GHS criticisms than most... which in this case actually makes me that much more confident that you won't be disappointed in the feel of the YC73.

As for my own experience... I think that pianos/EPs on the YC73 don't play great out of the box, but can be adjusted with the Velocity Offset and Depth parameters that were added in one of the updates, and that makes an enormous difference. Out of the box, the YC88 plays much better, though is a bit too heavy feeling at the bottom for my tastes... and at any rate, would be too heavy for me to gig with. And also, the YC73, while obviously not having as organ-friendly action as the YC61, is indeed surprisingly nicely playable for organ, as hammer actions go (and yes, better than the 88 for that purpose).

Jason's other mentioned scenario of 2-boards may also be worth considering. For example, a YC61 paired with a 25.3 lb Studiologic SL73 is actually a bit cheaper than a YC73, and would give you a piano-oriented hammer action and an organ-oriented semiweighted waterfall action. I've never played the SL73, though, and can't speak to how its action compares to the YC73. A nice extra benefit to this approach is that, for some gigs, you might decide you could get by with just the YC61, and then you have a super light 15.6 lb board to travel with! And while of course it's not the same as playing on a hammer action, playing piano from the YC61 isn't bad, as non-hammer actions go. (I didn't suggest a Yamaha for a low-cost lightweight 88 to pair it with, because unfortunately their recent models--like P225, P125, P121, P45--don't have 5-pin MIDI jacks.)