I am using a Microtech on my NA 3.4. They support turbo aps as well though. I went with them because there is a well respected tuner in my area to tune the car for me. Unlike TimT, I have no experience in tunning stand alone EFI myself. But, when I went to the AUTRONIC tuner local to me, he sent me to the Mictotech guys.....
As a side note, anyone stateside who is contemplating EFI should consider a system from Rudtners (timt and GTU935) as Tim has helped me out with a ton of stuff via email and PM. If I had a more local Link or Haltech tuner, I would have bought from Rudtners for sure. He gave me unbelievable "customer service" even though I was not a customer per se.
Just check out their website and look at some of the videos of the cars they have tuned.
Turn3 Autosport- Full Service and Race Prep
996 Turbo, 997 S 4.0, Cayman S 3.8, Boxster 2.7, Boxster S race car, 996. Cayenne TT