I would agree with you. But you have to be realistic. If you play GM on an XG instrument, it's not going to use the best available patched. I think on an XG instrument, you should play an XG MIDI file.
and on GS instrument, a GS MIDI file. All of them say GM MIDI compatible, but all that means is that they will use one GM bank. There are lot of things that you can't do with GS or GM, that you can do with XG. The most important being, using more than one drum kits.
Micco, you can download XG MIDI files from http://xgmidi.wtal.de/index.html
go to XG MIDI Library, XG Cover Songs ( you will find your favorite songs in XG format) and other peopels original stuff in XG format under XG songs...