There are some very aerobatic airplanes included with X-Plane, including an Extra, but none of the ones I've seen fly like what we're used to seeing in model aerobatics. It's really up to the model creator to make sure everything is right, and some of the guys who contribute models to X-Plane just hack them together.

Since I've used it with success, I think that if you build your aerobatic plane correctly in X-Plane, it will give you some worthwhile results. There are details that X-Plane doesn't model, so it's not absolute. It doesn't model all the effects produced by the fuselage, for example, such as lifting forces. Instead, it's up to the creator to input a drag coefficient, etc..

Still, for the price, it beats wind tunnel time at NASA. (!)