A rule of thumb is to have the same tires all the way around the car, i.e. if you still have the Pirellis on the front, put 'em on the rear.
Why is this, you ask?
It's because different makes, and even different models of tires within the same manufacturer have everything from their belts being ran differently, treads obviously being different, and entirely different carcass construction. This could effect you several different ways. It could affect you from say the front trying to track the rain grooves on the freeway, and the rears not. The fronts deflecting one way over bumps, and the rears going the other.. and when going through a turn, your fronts could react quicker or slower than your rears. This could actually upset your chassis to the point where you lose it. You could experience all of this, some of it, something else entirely, and you might actually not experience any of it. Chances are pretty small however, but circumstances play a part in it. Guys who drag their cars will a lot of times run drag tires in the rear, but they're going in a straight line..1/4 mile at a time. The slower you drive and the less you ask out of your car corner-wise will lessen the effect(s) as well.
'12 Boss-Competition Orange