the mustang would probably roast the porsche in a drag race because the porsche would end up snapping an axle from wheel hop or hooking up.
autocross it could be either car. the new boss blows the M3 out of the water around the turns. a boxster is really just the modern version of the 944. its a poor mans porsche. they arent really all that fast when you compare them to pretty much any other european sports car.