wouldn't it be better to do some print comparisons rather than crops if you really want to discern the difference? i realized doing so would be more expensive than just working with images on a monitor, but you'd be much more certain of your conclusion.
at any rate, i was a short-term user of the R-D1 so my print experience with its files is limited. depending on the printer and your crop, the R-D1 natively supports up to 8x12. i have done a little up-rezzing and feel that 16x20, depending on the subject, wasn't out of its reach. but the sweet spot is probably 8X10 to 8x12.
by the way, i've seen 16x20 landscape prints from the D30 that were really nice. something about that camera's files supported heavy up-rezzing really well.