Vegastang made some good points regarding the 3.08s. I have them in a 95 Cobra and havn't really noticed the things he mentioned, but then the Cobra is optimized for low-mid range torque- perhaps the GTs are different in that respect. The only problem I noticed is that 5th gear is not the best for mountain cruising- at least that was not the case before the twin turbos went in. On the other hand, the 3.08s and overdrive have been very good for freeway and high speed use- they provide good mileage and minimal wear on the drivetrain. I generally get ~330 miles per tank normally, better on long trips.
Just keep in mind that turbos need to work to produce boost- I believe the tall gears help to bring boost earlier and prevent unwanted tire spin to some extent. If you are going to use tall tires, get 3.55s, otherwise go to 3.27s.
The important thing is to consider how fast your engine normally runs most of the time and when your turbo boost comes on. The 3.08s give me 2200-2300 rpms at cruise (80 mph), I get full boost at 2300-2400, so the combo works well without having to shift down for a quick burst of torque. If the boost didn't happen until 2800, 3.55s would be preferable.