The SC-8850 seems to be the most complete GS document available that I have found. I use the Cakewalk Instrument File for the 8850 for all of my GS bank and instrument names.
For XG, I use a combo of sw1000xg and 9000ProV100.
SYSEX can be found in those manuals, but may vary from device to device, even from the same manufacturer.
As you mentioned, these are all old devices, but there does not appear to be any modifications to the specs since then, at least none I have found.