Yes, I think the roller clutch hub uses a bespoke freehub body that isn't compatible with others (since it's not the traditional ratchet-and-pawl design). I think Shimano likely made 8/9/10-speed versions of the silent hub, but it's probably not worth swapping.
I have done an 8-of-9-on-7 setup on my '97 750 before, and it works nice. That is, using 8 of the 9 sprockets of a 9-speed cassette...on a 7-speed freehub body. This requires either friction or 9-speed shifters (since the sprocket spacing will have 9-speed spacing), but it works well. The STX derailleur even works reasonably well, as long as you keep the largest sprocket to a reasonable size.