My buddy with an S2000 is goin to stick 245's on his rims that now have 225's on them, consider this..i have 245 45 17 how much wider could i go? for just the rear two rubbers
If you have stock 17x8" wheels probably the biggest you'd want to go is 275/40, but they'll bulge unless you use Nitto DR's, and 275/40's probably aren't the best for handling because of the bulge...255/45's still work pretty well, but your speedo will be off some...
Please keep in mind that if you oversize your tires it can be dangerous at high speeds. I don't know if you like to drive over 100mph but the tires can pop off the rims at high speeds. My friend owns a wheel and tire shop in NY and he has heard many stories like this. He refuses to install oversize tires on a rim if a customer asks. also, missouri_guy197 is correct in saying that your handling will suffer from the bulge.
If you not into high speeds and want the looks, then I'd say go for it, I wouldn't reccomend it. There's a reason why certain tire sizes fit a certain range on wheels....Safety.
I also have a 01 True Blue Convertible....I replace the tires with Dunlop 5000. they have a Lip Protection feature that makes the tire appear wider. Plus they are quieter than stock and not to mention the handling improvement on both dry and water.