I tend to agree with the observation that you need a GM synth that one the gives a full set of instruments. I think Native Instruments Bandstand is one such thing, although BIAB has some "delay" problems with NI BS. But if BIAB 2010 addressed the delay issues, the NI BS samples are really great.

The Sonar's Cakewalk TTS-1 which i think is a Roland VST plug in is also a good one, and works on BIAB better than NI BS, and sounds better than the Roland included in BIAB.

The samples in Kontakt and in NI's Maschine are great, and I hope you guys can find a way to make it work in BIAB.

On another note, the Real Guitar and Real Strat VS Plugins are good for the Guitar channels.