I would hazard a guess and say that +30 on a 18x8.5 is going to still out way too much.
Just realised that you have a 2012 WRX which will be the wide body model, I know that you can fit +35 in there with a bit of rubbing/guard rolling, +30 might be pushing it!
I did manage to find this for you, it appears that only the front wheel has been test fitted.
BBS CH Wheel 18x8.5 +30 fitment