The 1st gear ratio in the T5 is ~3.06 - just about like the 1st gear in a 4L60/E or TH700R4, and many lament that short/steep gearing.
Unless you want to run a much hotter/higher rpm cam in the turbo than stock, you would probably be better off with a 3.54 rear gear - like stock on the turbos. Turbos like something to work against to build boost.
A 3.90 would be totally wrong for a turbo with the size wheels we are normally able to run. Now if you tub the rear and make room for taller tires...