The Toyo R888s come in the same 245 and 275 sizes as the RA1s you've been using, so they should work just as well, if not better. The R888 275/35 rears are .5" greater in diameter than the 265/35 Cups (25.7" versus 25.2"), so the R888s should maintain the same profile look and effective gearing as the RA1s have for you.

I appreciate your willingness to experiment, but, with your wheels and good setup with the RA1s, I think I'd try the R888s next.