Whatever you load from an Motif XF file - if the file contains no sample data then this will not consume "any" of your 1.750GB sample memory. When you save off performances from Motif XF - if the performance references only internal preset voices then there will be no samples in your save file. Your save file should be very small in the case that you do not use custom samples. As opposed to relatively larger save files when your average samples are used.

"Full Concert Grand" and "Ballad Stack" are voices. If you made a user copy of these voices, they'd still be voices. Saving these off would result in an X?V file and those could have been converted with older firmware.

If you make a performance (on Motif XF) which includes these voices - there would still be no sample data saved in the save files. None of this should take up sample space in Montage. Given there is "without a hitch" compatibility -- I'm not sure why the converter would have problems finding the correct Montage single-PARTs to map to XF voices. 10 more days and counting...