I would say the LT headers and the O/R H-pipe. The Plenum and TB will do some but the LT headers alone will probably do just as much as those two combined would.
definitly the LTs and o/r pipe if you have the funds for it. dynos have shown about 12 rwhp and about 8 rwtq for the plenum/TB combo at most, whereas ive seen consistent LT dynos netting at least 15 rwhp. i think i remember someone getting about 23 rwhp from the LTs. i'll do a search for it and see if i can find it.
I would say the LT headers and the O/R H-pipe. The Plenum and TB will do some but the LT headers alone will probably do just as much as those two combined would.
The only drawback is that the plenum and TB are a simple 30 min do it yourself install, whereas the LT headers and midpipe scare me off. But if you have the tools, time and skills (or the money to pay someone else to do it), this is an easy answer - LTs and midpipe. Much more power potential.