If you want to do a turbo later then don't touch it until you can afford the turbo. Big exhausts KILL a N/A car. I had a 2.5" SLP loudmouth system on my mostly stock GT and the car felt incredibly soft off idle and in the mid-range. I switched back to the stock exhaust system and immediately the car felt more responsive and more fun to drive.
The only aftermarket exhaust I would probably recommend for a GT is the Borla, since it reduces back down to 2 1/4" after the resonator and 2" after the Y split. Velocity is everything on a N/A motor.
As far as the downpipe, you'll have to change that out when you go with the turbo anyway. No downpipe that bolts up to the stock rear manifold will bolt up to the downpipe of any turbo system.
