I'm guessing that you will be autocrossing this car in STX?
In that case you can't go wider than 9".
Although, however, your changed rear end gear ratio sends you into street prepared I believe (C Street Prepared to be specific). In that class wide wheels and big R-Comps are de-facto required. I also think you should really consider going with 17x9's rather than 18x9's, whether you de-mod to run in STX or run in CSP.
Am I correct that in Street Prepared that you can change the rear end gear as long as it fits in the original housing?
EDIT: Upon re-reading the changed final drive ratio throws you into Street Modified. However, wheel and tire rules for Street Mod are the same as Steet Prepared. I recommend 17x9's with a nice set of R-Comps.