I have been racking my brain over this issue the past week. Running an external tial 38mm gate with 10psi spring, boost cntroller unhooked until I find this issue, vac sourced from the manifold to rule out pressure drop from intercooler, and rest of the mods in my sig. So far have tested the cold side for leaks and it held 25psi. Leak tested inatke manifold. Found no leaks. Bench tested wastegate and it's doing what it is suppost to. No exhaust leaks.
Part throttle it builds 10psi like a champ, go full wot and the pressure imediatlly drops to 7psi and car feels like it's loosing power.
Called precision and spoke with a tech and they seem to think the .63 ar is the problem and I'm running into back pressure issues. My exhaust pre and post turbo both flow very well. Cam is a little big for the turbo. 218/224 114* .580 lift. Springs are new style comp 26918 lsx behive springs. 6262 turbo has new cea turbine and is buick housing. Personally I dont think the issue to be bp although I havent logged that yet.
I can post a log but to view it you need to download sct livelink.