Thanks for the question. First, the Yamaha CFIII (CF3) is a different acoustic piano model from the Yamaha CFX. You can learn more about the Yamaha CF Series HERE

The CFX is the current top-of-the-line acoustic piano and has a different character from the CF3.

The thing about acoustic pianos and samples of acoustic piano is that just as no two pianos are exactly the same, it is rare in sampled products that a programming team wouldn’t make adjustments to the data for the specific product. So even if the data is culled from the same sample session, you are pretty much guaranteed that the programming is done on a per product basis. They certainly are not (ever) exactly the same. When Yamaha creates samples of a musical instrument, the sample session may include a terabyte of data... from that sample session programmers can draw the data that they need for the product they are working on. The sample data may wind up in a small portable product and in a top-of-the-line synth or stage piano... and many in between... but it will be treated (edited) differently in each. Some products will have but one velocity layer - other may have multiple velocity layers across the keyboard... still others may use the information gathered from those samples in a completely different manner.

I know this does not help you, but you should make your decision based on the sound and feature set that best fit your requirements. These are different product lines with different user interfaces, and technologies.

The CP4 STAGE has some component modeling (SCM+AWM2) which allows more/deeper customization of the sound. It has the ability to dive in and work on acoustic and electric piano construction.
The CP88 has an interface that is hands-on, you can quickly customize things in real-time. The access is different from the CP4 Stage.

If you were to spend time with each, you can decide where you feel most comfortable. As to whether the Spectral Component Modeling approach suits you or the real-time hands-on interface appeals to you - you will have to decide. The sound? Again, you will have to decide. They’re Yamaha, they are going to sound good! You will need to decide which one sounds best to you.

I simply prefer the CFIIIS on my CP4 and CP300 to the CFX sample on my CK and YC. Is there a chance that the CFIIIS sample could be included on future CK and YC updates?

The CP300 and CP4 have different tone engine options. The CP4 uses SCM (as mentioned before in this thread) and could therefore not be replicated on the current YC or CK products. The CP300's CF3, as a pure AWM keyboard, "could" be added. If there's the Waveform ROM space available to do this or if Yamaha will do this is anyone's guess.