+15 offset in a 6.5" wheel will be fine.
I've currently got +20 which IMHO sits inbound a bit too much. But nothing rubs.
The new RS-8's I have are a 0 offset .. mmm
Go the 15" I reckon. But thats just my opinion. I'm not a big fan of large diameter wheels on AE86's or any oldschool car for that matter.
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