boost falling off please help

[xsephish]

Fully built redtop hks 264 step 2 cams. Here is my issue I am running a gt2860rs on wastegate no boost controller at all it would spool up to 20psi at about 4200rpm then boost would start falling off and level out around 15psi.

My turbo elbow is one of those stupid isis screamer pipes not by choice but its what i had and was over budget. At the suggestion of my tuner we added a turbo smart e street series for the boost correction feature to prevent this drop in boost. It is a very reputable shop he has set these controllers up before but it isnt fixing the boost drop we can add boost but same thing happens. Also it is very consistent on the boost falling off. You can look at the boost log on the dyno sheet from wastegate and EBC set to 24 psi with correction and its the same full spool and the drop off lines all sit on top of each other if it werent for the 24psi spike vs the 20psi spike you wouldnt even know it was a different run.

My theory and where i need input is this. Is the screamer pipe my problem i know they dont seal all the way up to the back housing so am i creating such an exhaust leak by running that elbow that it is causing this issue? Someone please chime and and help me on this or atleast help me brainstorm issues of boost falling off. There is no boost creep it goes as high as we set it the dyno the EBC (peak boost recall feature) and me watching the gauge in the car all look good its this damn fall off from about 4.5k to redline.

Also at idle the screamer pipe is sucking in so much fresh air that my wideband cant read driving its fine and on the dyno AFR is good but just a side note.

added dyno graph

green= old ebay top mount t67 MBC 19psi
red= EBC with corection turned on and it looks like its fighting past 5.5k to hold 17psi
blue= internal WG only

blue and red both you can see 4.5k im fully spooled and about 4.7k it just starts dropping. I am happy with the numbers car is a blast to drive very responsive, just trying to get some more minds on this pressure drop and see if a screamer pipe could be culprit

Also it was really hot and humid out last night and that last pull was on a hot motor IC and hot water temps