Rear wheel drivers can be hard to handle and can get away from you fast if you don't have tight reins on her.
All wheel drive will always beat rear wheel out of tight corners, assuming the power and weight is similar.
In real life with high power AWD cars, you can't just dump the clutch and slam the gas like you can in a RWD car. In a drag race the RWD gets the advantage early on, the the AWD has to make it up with power.
There's nothing that says RWD cars get more power than AWD cars. Just because there might be more powerful RWD cars, that doesn't mean it's because they're RWD. The differential setup has no effect on power except that AWD cars can put that power to the ground more efficiently.
This is true in extremely high power outputs, but not in daily driven sports cars like the Mustangs, Camaros, and other similar set-ups. The Audi, Subaru, and Mitsubishi put all four down and have much faster reaction off the light. The rear wheel cars tires when dumping the clutch spin like mad because those pathetic tires can't handle the car. My RX-8 has 310 hp, and I always play catch up to my buddies stock STI. My mustang had the same problem. I think that in the game they should show a much greater difference in traction and performance depending on tire width and grip type. Hopefully it's something they will add in the upcoming game, for right now in PS the customize option is purely cosmetic and even more disappointing to me.