Hi Kevin,
I can tell you first hand that the Korg PA-80
s Acoustic Grand Piano leaves a lot to be desired imo. I find the Yamaha PSR 2000's Grand Piano sound to be much better and more realistic sounding. But then again I didn't tweak the sound at all so if it's tweaked it might sound better. But like I said the Default PSR 2000 Grand Piano is much better imo than the Default Korg PA-80's. And after tweaking my PSR 2000's Grand Piano it is even better sounding. Yamaha did a great job when they sample WAV ROM'ed the PSR 2000's Grand Piano voice. To give you an example, I played the Tyros extensively a while back and compared it side by side with a PSR 2000 using a pair of Sennheiser HD 280 Pro Headphones-(AES="Audio Enginnering Society" 2002 Innovation Award Winner!) I consider the Tyros's Grand Piano having just a slight edge in sound quality compared to the PSR 2000. And I think some of that slight edge is due to the Tyros much better Key action, ie., better Key response/action= improved sound output/nuances. That gives you an understanding at just how good I think the PSR 2000's Acoustic Grand Piano sound is especially if you've heard some of those Tyros Grand Piano.mp3's floating around the Internet. On a side note I hope the Korg Pa-X has an awesome GP patch. I can't wait to get a hands on demo of it. Maybe Yamaha has something up its sleeve too!

Best regards,
Mike