Thanks a lot, deemster. This is fantastic. I was downloading ever larger and larger soundfonts to try to get the best sound and was still not satisfied. All that I really care about is accurate reproduction of MIDI music from games from the late 80s to late 90s, so those huge soundfonts were extreme overkill and still not totally accurate. Your little 3MB SC-55 soundfont sounds better with that old game music than even the 250MB soundfont that I have, and is just what I've been looking for. I think that I'm going to delete the 500MB of soundfonts that I downloaded and just use the SC-55 one, since it's much smaller and better for my needs.
The only thing that I'm slightly disappointed about is that the DX50XG one is, as you noted, meant for GM playback, not XG. I tried it with a few XG MIDs and, as you implicitly warned, they sounded pretty off. That soundfont sounds pretty good with GM, but not really any better than the SC-55, so there's little reason that I can see to use it over that. The S-YXG50 VSTi sounds perfectly with XG MIDs, so I can use that when I get nostalgic for my old DX50XG, but it'd be really nice to have that in soundfont format, so that I don't have to have two different MIDI drivers installed. I don't know if it's possible to convert or port that to a soundfont.
Anyways, again, thanks a lot for the SC-55 soundfont. I wish that I'd found it earlier, before I spent lots of time downloading and testing various other soundfonts. I'm off to delete 500MB of soundfonts that I don't need anymore...