You say that like all wheel drive is a bad thing. This may be your opinion but for all but the most evocative of performance cars RWD is a little overrated.
How many people out there even have an inkling of the true benefits of RWD? The crisp steering feel, unencumbered by the interference of driveshaft forces. Or what about the delicate change in feedback as the vehicle pitches under acceleration and braking. Or how about that wonderful lack of understeer as you turn the car in early, balance the throttle and mash it down on corner exit? We've heard it all before, but in everyday driving you'll never approach the limits of your vehicle to experience this. This is the stuff of highly illegal road use and track day pursuits.
Oh, and all of the above could just as easily have been written for an STi, EVO or R32 for that matter. AWD cars are getting better and their tactile rewards are on the increase.
Engine placement is a key consideration for a vehicle that has drive sent to the front axle. In fact, it's fundamental to the design of any all-wheel drive vehicle. Weight over a given axle is essential in increasing grip and the way a vehicle transfers this weight during acceleration and cornering has a direct effect on the amount of traction at that axle. That's why RWD vehicles are better at accelerating than their FWD counterparts: weight is transferred aft under acceleration, increasing the grip at the rear giving better traction for faster pull aways.
So, now that an AWD vehicle has a measure of drive sent to the front wheels, it's important that there is enough mass over the front axle to provide traction. If the engine was mounted well back then the lack of weight over the front axle would negate the benefits of proper all-wheel drive under full bore acceleration.
There's more to AWD than meets the eye. For those of you that haven't, I suggest that you go and drive an Impreza WRX or EVO or S4/RS4 and marvel at the levels of grip that each of these vehicles has. It makes for some truly phenomenal cornering
ability. Fact, not fiction. If you're unconvinced then read up on how the R32 has trounced the lap time of the 11 kW stronger, mechanically "more efficient" 130i in every comparison I care to recall.
Oh and as for the TT's engine being mounted over the front axle, well, the Z4 Coupe's is mounted more "over the front axle" than "behind the front axle" too: