I was getting off the interstate today and there was a dark blue E39 BMW M5 (400hp, V8 version) just ahead of me. He was driving pretty aggressively and I decided to see just how close I could stay with him in my stock '06 WRX TR...
At the end of the ramp he made a left onto a 55mph two-lane highway and I followed in hot pursuit. I stayed a conservative 20yds off of his back bumper and was able to maintain this distance as he accelerated, but I could tell he wasn't going completely all out as we were coming up onto a van travelling about 60mph pretty quickly. I stayed on his tail for about 2 miles until the van made a left turn into a gas station. As there was oncoming traffic, we came to a complete stop until the van was able to make the turn.
The dude in the M5 floored it and I could hear his traction control kick in and see black soot coming out of his tail pipes. I floored it as well and stayed right on his bumper through 1st and 2nd gears. When I shifted into third at ~65mph, he started to walk away from me slowly and we eventually shut down at 75-80mph.
I did a little research on the E39 M5 when I got home and saw that Car & Driver got a 13.4 @108, Road & Track saw 13.3 @108 while Edmunds got a 13.75 @105. Seems like the top end of these cars is simply amazing, but in the 1/4 they're about as fast (in terms of et) as a stock STI/Evo.
I felt pretty good about well my stock '06 WRX TR kept up especially since I took her to a 13.74 @98 last weekend at the dragstrip.