Well, as it happens I just test fitted my 275/60-15 Nittos on the new wider rims with more backspacing. Took a few pics so that I could better compare.
Here is what I had:
Centerline AutoDrag 006 rim, 8" wide, with 4" backspace (-13 mm offset)
Here is what I went to:
Centerline AutoDrag 005 rim, 8.5" wide, with 4.87" backspace (+3 mm offset)
The DRIVER side is the NEW rim, the PASS side is the old rim.
I will say the following: it is pretty tight in there...very close to the leaf spring and any bigger of a tire would most likely rub against the inner guts of the wheel well. I was considering going to a 286/55-15 MT tire, but that'd need an extra 1" room, and even with 0.5" split per side I think I would have a problem.