I'd wait until the turbo kit is complete, to see if the addition of an underdrive would affect things adversely.

From the feedback I've seen on MB-World, the UDP kit is good for a decreasing amount of HP as you go higher in the gears. Essentially, the load is greatest in lower gears, and the UDP kit's effect is most noticeable in gears 1-2-3. Something like 10hp in 1st gear, 8hp in 2nd, 6hp in 3rd and stays at around 5hp in all higher gears. This is actually a good thing, as this is where the Limited struggles most IMO. As long-legged as the car is, once it's moving it was never a slouch. It just struggled to get out of the hole, as evidenced by my 2.3 60' time (with no traction issues).

By itself, I'd say the effect of the Evorsport UDP wouldn't be that great. Combined with intake, tune, and a de-cat... it might put a bigger grin on your face.