Not in this case - you keep it within specs and you're good to go. It needs a proper tune tho! Good tunes are very easy to come by - there are a few guys who will email you the files!
The clutch may not last as long though!
Remember, 996TTs have the Le Mans 911 GT1 engine in them. They're bullet proof as long as tuned properly to 800 or so HP, maybe 1000.
I dont like the GT2 idea so much but I love the GT3. The difference between a GT2 and a Turbo is the turbos (which are not worth paying much for), intercoolers (not worth paying much for), lack of AWD (if you want RWD, the conversion in a turbo is FREE, you remove a single shaft and you have it), no backseats, seats, some exterior body things, wider rears, ceramic brakes (which you dont want), clutch and slave, plus some minor things - BUT you get a GT2!!!! Its special. If you want special, it may be worth it - but itll be $80k+.
A base turbo with a flash is going to be way faster and much more usable than a stock GT2. The GT2 you're going to want to keep pristine too - you'll feel fine enjoying the Turbo.
I think that a GT3 is the car for you unless if you're like me and you want a highly usable car - then get a Turbo.