You need to match the convertor, turbocharger, and engine to minimize lag. That's going to mean running a slightly smaller turbo than you would normally find recommended here, with possibly a slightly higher stall.
From a purely performance on the course aspect, the lockup doesn't matter. You'll rarely be going fast enough at part throttle for it to engage, but you'll appreciate the boost in gas mileage and it cuts down on heat in the transmission when cruising.
I put new headers on simply for reliability. My stock manifold was constantly cracking on the driver side. It just got to be too much of a pain constantly pulling it off and having it welded back.