We had a shoot-out of the piano sounds over at Roland-arranger.com (or it might have been the old G70 Users Forum, I can't pin down how long ago it was) quite a while back. You are right, it was generally held that the Classic piano was the same samples as the GrandX in the G70, but the programming had muted it. Also, in the E50/60 version (possibly E80 too, I can't remember), the samples seemed shorter, or more compressed.

Your solution seems to be the one we all agreed came closest to the G70's great piano sound, although personally, I'm not sure you need to wind the cutoff all the way open. Play quietly, softly, and see if it closes down enough. There's a delicious warmth to the sound of the G70 piano at ppp-mp, that you might lose if you open it out too much. You might also try winding down the resonance parameter a bit, that might help somewhat.

I tend to use my G70 piano at Medium touch response, it seems (on the G70 keybed) to give the best range of sound, without having to pound the dear thing too much at ff-fff Depending on your touch and the E60's keybed, YMMV...

BTW, if you want a nice bright, almost Yamaha-esque piano sound without going to one of the tinny Roland pianos, try using the Part EQ, but rather than jack the treble up a bunch, just do it a little, but cut the low end substantially, then raise the whole Part Volume. Instant cut through the mix!

Hope this helps.