Mustang
All the power, cornering, peformance of the M3 but cheaper than a 370z. They did a head to head with the stang vs. M3, aside from interior the Mustang was just as good as the M3.
If thats not your cup of tea I would go 370z but I wouldn't boost it. If I wanted that much power I would just go with a Vette to begin with. A boosted Z is fun but too much of a headache for me. I like modding but when you are having to get that extensive to get the power you want, in my opinion, you might as well just get a more powerful car.
I Say no to the GTR for one reason: no manual tranny option.