Well, this is all preference, but here is my 2 cents: First of all, I assume you will be running in STX based on your wheel choice (7.5" wide). STX means that you will be running DOT tires, not R-Compound tires. With a 225/45 tire, you will be running a slightly taller sidewall. This (combined with the more flexible sidewall of DOT tires) will likely create a more "squirmy" feeling and response as compared to the 215s. Now, please take my advice with a grain of salt. I haven't tried 225s on the Civic SI; However, I experienced this on my Mazda MP3, lost some responsiveness, and ended up regretting the larger tires.
Do you really need the extra 10mm of grip, or wheel protection of a 225? That is up to you. Usually tire type will have a much larger impact on traction than 10mm will make.
Will 225s be bad? No. As a matter of fact, I think they look better than 215s.
Can you be quick on them? Of course. But in direct comparison, I would likely prefer the feel and responsiveness of a 215 over 225.
Either way, if you are only considering a 7.5" wheel and a 225 tire, i honestly don't think you are looking to be competitive in STX at any more than a local level. (no offense towards you or your driving; its how our cars class). So I would examine what your other goals are: Looks? Wheel protection? Comfort? Response? etc.
Personally, I am in a similar situation as I think you are in: Full time daily driver with occasional autocross (I actually even run my autocross wheels and tires on the street during the summer). For me, my first priority is having great feeling/response. So my plan is for running 215/45/17 Michelin Pilot Super Sports.
Hope it helps!
