Bigger diameter wheels (19" and up) will actually hurt handling somewhat, but probably not enough to notice on the street, because they are heavier. Also much more prone to tire damage w/ the short sidewall.
For handling it is better to put a smaller tire on a given rim, rather than having a loose/baggy sidewall that will let the tire roll over more (but this may give a slightly softer ride - the 245's are max for the 8" wheel). The MPSS is apparently very prone to this rollover edge-wear, but I'm a Conti fan and haven't tried them - my ExtremeSports held up well for my first day of track use (but if you can actually drive, you might put a lot more wear on them than I did
).
And you might want to check out some of the 17" options if you plan to drive hard, track, or have bad roads. Lots of track cars use 245 on 17", and can clear the M-brakes OK - to me these don't look much different from the 18's. I think there is a set for sale in the NY/NJ regional forum to see how they look (and probably many others in the picture forums).