Rikki, you should find the XG drumkits in Bank 127 and the regular GM/GS drumkits in Bank 128.
I can see that you must be very particular in trying to get the soundfonts to sound identical to the Yamaha XG instruments. I don't think you will succeed by trying to change velocity or volumes . It is more than that, e.g., harmonics, envelops, modulation, etc. In addition, Yamaha would have used different (their own) drum instruments then what you will find in any other available soundfont or other drumkit.
I would strongly recommend you stay with Yamaha hardware to do the job for you. Even the best drumkits or other virtual acoustic instrument out there today will not satisfy your needs as near as I can tell. If you want the Yamaha sound then Yamaha equipment is the only way to go.
Light weight alternatives might be Yamaha sound modules, e.g., the MU series. If you don't mind the high latency you could also try the Yamaha softsynth. There are soundcards out there that use the Yamaha XG chip. I think for these you need a regular computer.
I apologize for sending you down the wrong path. Sorry.
[This message has been edited by Frank L. Rosenthal (edited 09-26-2004).]