I have done some basic testing with a Marconi 2945 communications test set to compare the IC-7700 with the IC-7800MKII. There is virtually no difference between the receivers in the two top shelf Icom flagship radios. The S-meter sensitivity, Minimum Discernable Signal and close in blocking performance seem to be almost identical. The IC7700 uses later generation DSP processors that are faster but in general terms there seems to be virtually no difference.
I wanted the IC-7800MKII for the second receiver mainly. It's nice to have a completly second receiver running on a different antenna. I can be working 20m and keeping an eye on 10m or 6m for band openings. Also running split in CW while keeping an eye on the DX and his pileup. It really is as good as a full SO2R setup!
After owning the IC7700 since May 2008 I can safely say that it is every bit as good as the IC7800 but with only one receiver.
73 and see you on the bands.