The S-YXG100 in theory will understand the more of the more advanced SysEx messages (they seem to have a few more Variation Effect parameters than the S-YXG50), but for FF7 purposes that is a moot point, the since the game's XG soundtrack will play back correctly on even the S-YXG50.
However, I'll say it again, when I tested the performance FF7 had when using the S-YXG100 in comparison to its previous performance using the S-YXG70, I did notice the S-YXG100 did respond faster than the S-YXG70.
And, yes, I totally agree that the XG soundtrack really does bring FF7's music to a new level. My only gripes, though, are the fact that it doesn't have a French horn sample that sounds like the traditional Squaresoft one (the one found in the Soundfonts), and the fact that when using the XG soundtrack, OWA is only an Instrumental, which means no lyrics for me.
Oh, and one more thing. In case some of you are envying the fact that I can use my XG hardware synth in-game, let me tell you, that compared to the softsynths, my hardware doesn't have any bass emphasis. However, I suppose that that 2.1 subwoofer system I got for free a few month's back could make up for it, but still, I would have prefered the enhanced bass that the Softsynths seem to have.