crazy power loss

hi

i am tuning a 2003 wrx running a gtx2871 turbo and link g4+

98 fuel with methanol injection and the usual full 3 inch exhaust, headers and external wasteagte

built engine with stg 2 cams

lambda is 0.80

ignition at peak torque is 14 going up to 19 at higher rpm

boost is 1.6 - 1.55 bar

here is the dyno

after 6000 rpm we are loosing 100 whp

what could it be?

It could be many things, as it's worse at higer rpm and hence gas flow, I'd be wondering if there was an excessive build up of exhaust pressure?

tested without an exhaust and without midpipe same thing

tested without intake same thing

weird