From what I've read over the past few years typically when a car is rear wheel drive it loses approximately 16% from the flywheel to the wheels, front wheel drive is 14%, and all wheel drive is in between the 28%/32% I've heard variations.

Usually on a Porsche what is the drivetrain loss like? For the 911TT or a C2.

 

App. 18.5%-20% for AWD, and 12.5%-15% for RWD.