I have a 2007 Mini Cooper S r56 automatic, stage 2 by VRtune, it’s been giving me this really, REALLY annoying problem where whenever I’m trying to push it 2nd gear perfect, 3rd gear perfect, 4th gear good, 5th gear holds boost perfectly at 17-18 pounds until it hits 5000 rpm, once it hits 5000 rpm boost drops drastically to 14 then 13 then 11 and feels extremely sluggish for the rest of the gear until I let off the gas and shift up into 6th and get back on the gas. It sometimes also does this in 6th though at lower rpms but then will rise back up, I’ve been breaking my head trying to figure this stupid problem out, literally loosing sleep reading non stop on articles with similar symptoms but none exactly like this.

I also want to note a couple other things that maybe might be the culprit:
-realized the other day that I have a blown downpipe to turbo gasket and a crack in the welds of the downpipe(idk if this can be anything)
-valve cover has a leak

mods:
Cold air intake
Fmic
turbo piping
turbo smart bov
catch can