The 345 width won't be a problem for you, but the MT's are 27.5" tall, so if your car is lowered, you could possibly run into rubbing problems due to the height.
I just picked up a set of the MT 345/18's and had them mounted on my stock rims. I got them mainly for the drags and I haven't put them on the car yet, but they sure look like they ought to hook well. Very soft soft compound, so I can't image them lasting very long on the street. Also seem like they would be picking up a lot of gravel on the street.
I'm not running enough power (600RWHP) to warrant running them all the time, plus I like the twisties as well and we do get rain here in Oregon. Unless you are running a lot of power or only play a quarter mile at a time, I'd stick with something that is more of an all around tire. I think the PS2 is hard to beat, though I hear a lot of good things about the Nittos as well.