Just a quick correction Christoph, It was Tiger the killed SFS, as Apple changed Core Audio, which broke SFS, causing GB to crash anytime an instrument accessed SFS, As I understand it, the developer gave up re-engineering SFS for every OS upgrade. As I had purchase a license not 2 months before upgrading to Tiger, I was understandably, VERY disappointed that I could no longer use SFS.
Although, since I did not actually have GB '08 installed prior to Tiger, I can not say whether SFS was still compatible with GB '08.
Anyway, as you state, you CAN use SF2 files with GB as long as they are only SINGLE instrument soundfonts - I have several installed. However, if the sf2 file has more than one instrument (as has the Quicktime MIDI sounds), you will only be able to access the FIRST bank of sounds. This is because the Software Instrument Track does not send Program change information, which you need to change the bank.
Sorry, I don't know of any SW that produces Genesis/Mega Drive sounds. You may be able to find the individual program sounds as standalone sf2 files though, which you could use in GB.
Hope this helps clear things up.