I'd suggest two things, piano-sound-wise: 1st, Korg offers a (admittedly pricey) PCM card with an acoustic piano wave set that's a vast improvement over the original. That's no doubt the result of manufacturers having to cram "everything" into a new synth, leaving only x amount of room for the piano sample, which, as you point out, often ends up being weak.
Second, by laboring over it a bit, I managed to get a pretty potent acoustic sound going in a combi. Of course, that may be a mere stopgap for your needs, but I like it pretty well. Layer the A01 & B01 pianos with a string sample ("The Strings" is a good choice) for a little more oomph and tweak the reverb & exciter FX. That may get you close, at least. I've found that about 4 layers leaves me with adequate polyphony for everything but Rachmaninoff! Good luck!