YouTube - Nissan 370Z vs. BMW 135i, Mazda RX-8 R3, and Pontiac Solstice GXP - Car and Driver
370Z beats the 135i stock vs stock and is $4000 cheaper. The straight line accel is identical and the 135i has more low end torque (duh) but the 370Z is the clear winner in this comparison. Also, I've been doing some reading and if you just do an ECU tune and turn up the boost on the 135i it wont perform nearly as well or as reliably as if you added supporting mods like FMIC, exhaust, fuel pump, etc, which will bring the modding cost over $2k. So, yes, a bolt on 135i with tune will make close to 400 whp, but that's total about $6 grand more than a stock Z. The point? If you make the cars equal in price, that is invest $6 grand into a stock Z just for power (SC or TT), the Z will still make more power and have the superior handling if left stock in that dept.
They're both great cars for sure, but in independent testing, the Z wins and comes in a lot ($4000 to me is a lot of money) cheaper than the 135i.
HOWEVER, if all you want is performance at a price, you'd be better off with a 5.0 Mustang GT. So performance on a track and in a straight line is surely not the only criterion for selecting the car for you.