I have a TPC turbo 2007 Boxster S that I'll be selling to make room for my GT4 so I may have a bit of a unique perspective on this. I haven't yet driven the GT4, but, from reading the reviews of other posters, it seems like the GT4 could certainly benefit from more horsepower, particularly with the tall gearing. That having been said, I wouldn't recommend putting a turbo in the GT4 for the reasons stated above. (At least until it's out of warranty!)

However, I think the others who posted above are more than a bit too negative on the use of turbos in the mid-engined platform. My TPC turbo Boxster S is an absolute blast to drive (and track) and, in my opinion, is WAY better than a stock Boxster S. If you asked whether you should put a turbo in a 987 Cayman or Boxster, my answer would be absolutely. (The turbos work better in 987.1 Boxsters/Caymans because the turbo kit has larger injectors to spray more fuel...but those engines are admittedly a bit more fragile so it's a tradeoff.) I just wouldn't do it to a GT4 since the GT4 is such a special, purpose built car the way it is and it already has more power to begin with.

If you do decide to install a turbo, I would strongly recommend also getting methanol injection. The methanol adds a bit of horsepower but more importantly cools the engine so the two work really well together. If I drive a stock Boxster S after my TPC car, it feels really anemic and, to be honest, one of my fears in getting the GT4 is that I'll be disappointed in the power...but I'm sure I'll get over it.

With respect to reliability, I have 15,000 "turbo miles" on my Boxster with around a dozen track days, and I've had no engine problems yet, but I can't say that there's no extra stress on the engine from the turbo. I do look behind my car once in a while expecting to see a vapor trail of white smoke, but so far so good. There are definitely guys that have driven their TPC turbo cars for tens of thousands of trouble free miles, but I wouldn't get the turbo if you're not prepared to void your warranty and possibly grenade an engine.

If you have more specific questions on the turbo option, Planet 9 is probably a better place to ask. TPC is a sponsor, and more guys with turbos post there.