My rear XP8's are virtually new but I need new fronts.
I'm running hoosiers and the last set of XP10's only lasted 2 days of CMP

I was thinking of trying XP12's but will it mess up the brake balance running with XP8 on the rear?

 

Mess up compared to what you have? I guess it'll mess it up if you like what you have.
You'll get less rear brake bias from what I've found and if you don't mind that then it won't mess it up

 

well the only experience I have driving a car with a weird brake balence was a S13 240 with G35 brakes up front and stock at the rear on an auto-x

It was horrible and I locked the fronts and was impossible to trail brake

I don't want something like that

 

I don't think it will be an issue. The 10s are very similar to the 8's the only issue would be if you find that you are overheating the rear pads.

10s have a very similar cof compare to the 8s. The 12 is a big jump though!

I was told that the xp12s wear quicker than the xp10s and from looking at my pads it seams they wear about the same rate if not faster.

 

If you're braking hard at CMP, 2 days is pretty good. Talk to Carbotech, the xp12's will wear even faster. I had the same thoughts exactly after my first time there with xp10's. It's just a part of running at CMP, it's very hard on brakes.