Hi Lee,

I can't answer all your questions but I can say for sure that points 2, 3, 4 are definite certainties and they sound perfect as well.

Both Genos and T5 styles (including the audio styles from T5) sound great on the SX900 with no tweaking so far on anything I've tried, unlike how awful and sometimes unusable the Genos styles sound on a PSR S970/S975 for example (even after tweaking), so the SX900 certainly has the most up to date engine for compatibility.

All previous models from T4 below are perfect as well.

Midi pads work fine as I've been using some T5 ones and work great.

Not sure about Audio ones though as never used them.

If you're wanting to use any expansion packs from the S970 onwards, then they all work as well.