Jiko and CG were amazing dining experiences, IMHO.
But I think you're right. As far as early vs. late ressies, I found that it didn't really matter when dining at the resorts - for select restaurants. For the most popular ('Ohana, Boma, Whispering Canyon, etc), eating early is the way to go.
With the Signatures, I did find that it was just as easy to get into the restaurant at 8:30pm as it was getting in at 5:30pm.
Good luck!!