I have a 393w that was NA and running great on the street, had a custom cam and made the jump to a twin turbo setup, I recommend going with a custom cam from a well known person, I am running a Pro-M EFI on my car, with that being said here are some of my issues going turbo:
1. Fox engine bays are small, 351w
based kits really enhance that, be prepared to clearance, cut, hammer and hack your way to make things fit.
2. Turbos work better with automatics, I sold my A5 and picked up a 4R70w, its full manual shift and looking forward to driving it.
3. You'll definitely want to run E85 with your compression ratio, I'm at 9.9-1
4. Heat is always an issue with Turbos on a car that wasn't designed to have a Turbo, be prepared to address this during the build not after, of something's close, shield it, cover it or re-route it.
5. Hope you have deep pockets...