Is this a daily driver? If so, you might want to consider getting the same for front and back. Reason: if you get a blowout or a flat on one of the rears, and even if you have a full-sized spare on hand (not the OEM donut), you DO NOT want to be running a mismatched pair of tires on the rear, even for just a few miles to get home. Burns out the clutches in your limited-slip differential.
So, with big fatties in the rear and skinnies in the front, you'd either have to rotate your fronts to the back and put a weird-looking fatty on one side up front (which could compromise your steering ability) or you'd have to carry around a big huge monster spare in the back of your car at all times.
I'm facing the same dilemma, myself, as I'm running 15's, too. Want more contact patch but don't want to make it impractical. You might want to consider just going with 235's on the front and the back. You could go with 245's, but I doubt that they'd handle very well - too much sidewall flex/bulge, especially on the fronts in corners, and you'd risk rolling the tire right off the rim in a hard turn.
On the other hand, if this is just a weekend-only or track-only beast, I'd recommend going with something like 205's in the front and 255's in the rear. (Currently running 215/60/R15's on the front/225/60/R15's rear that were already on my car when I got it.)
