Between the M3 and the M4, I would go with the M4 in large part due to the increased torque, lighter weight, and vastly increased modability - that alone with get you closer to the sub-4 second mark which seems to be an important factor for you.
But that said, if tracking or sub-4 second time is important to you - I would go with the Porsche (GT3 over the turbo if you're going to be a tracker - or stick with the turbo if you don't intend to track and it will be a road machine). Its one thing to take a car and spend a lot of money modding it to get it where you want, its another thing altogether to get a car that properly begins at the performance mark you want. Plus the new GT3's look stunning. (plus you haven't had a P-car yet as one of the earlier commenters mentioned).
Nissan GTR is nice, and its performance hold its own with the above-mentioned porsches - but its not terribly good looking, its interior is sub-mediocre, and its still a nissan at the end of the day. Lot of bang for the buck, but if money isn't as much of a determining factor, then there are better performing all-around cars, for not that much more.
Can't speak tot he SLS since i'm not familiar with its figures. Historically even MB's performance cars leave something lacking on the track, compared to other similarly priced cars. But they're fast street machines.
On balance, I would go with the Porsche. Echoing the sentiments of the previous posters, I'd go with a GT3, over the turbo.
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