The PSR2000 L/R output (which sums the two sides) definitely loses something. I don't think it would be dramatic though. I extensively tested the Tyros 2, switching between the Bose L1, with both outputs running into it, and two Barbettas. There WAS a dramatic difference--the Bose sounded much better. I also used HankB's PSR3000 into the Bose and I definitely heard phase cancellation on some voices, such as the Grand Piano.
They changed something on the T2.
This is getting away from your situation just a little, but the principle is the same.
Mono in itself sounds great. Stereo pans some sounds left and others right. Sounds great if you are sitting where you can hear both sides. I believe the problem is in the way the keyboard mixes the sounds into mono. Some do it better than others.
DonM