I'll let the experts have the final say but if you already have test pipes I can't imagine LTH would give you anything worth the effort...unless you do the work yourself and enjoy it. On the other hand if you want every single bit of HP you can get; period. Then sure.
I also don't think the underdrive pulley is worth the effort either. I scaled -published- company dynos and you can see that what HP gains they advertise are real...at the very top of RPMs. Down the line; zero.
So test pipes, F.I. cat back, and Stillen Gen 3s...move on to forced induction or things like coil overs and sway bars; IMHO.
And post some pics!