I've heard ALOT about this topic on all of the boards.
A couple of things I have found out.
1. It IS possible to mount up to a 12.5 on a 8" rim, however as stated by you AND the Tire guys, I feel it is NOT a good idea. It can cause premature wear, and is also more likley to "pop the bead".
2. The back spacing on the rims are different, and you will get less rub with a wider tire in a wider rim. There will be no signifigant buldges on the sidewall, and will be able to have full turning radius with no rub.
When I was looking for tires and rims, I did tons of research on this subject, and the conclusion was always the same....when you go bigger, you need to go bigger. I went the 12.5 Yoko's, and put them on a 15x 10" rim. Best 10 $ decision I ever made.
Rob
:P :)