give you an example of how that backpressure will kill a cam’s range. the ls6 cam which seems to be very popular pulls na till about 6200 but in every boost setup ive seen ti starts to choke at 5500 and drops badly at 5800. sorta what yours is doing. more duration would allow you to pull much higher.
im kicking idea of going from 2.5 off the discharge to 3" charge pipes instantly and then of course going 4" to the tb. this should create less IATs and allow the spool range carry little further. i do plan to do dual 3" with these turbos i know i can fit 4" dp on the one side but doubt i can do that on driverside, however i may even do 3.5 but will see. going to see if i can step up to a large AR. should be interesting with a 3200 stall and 410s.
heres a thought if you have time to do this. get another boost guage mount it in the IC before the core then take reading at intake for other reference. see how much drop you are seeing. if you see more then i think 2psi drop that can also be causing your power range loss. some say your dual 3 into single 4 is causing you issues but then others say its fine. changing it out cant hurt and can only help. simple solution would of been toss cut outs on the DPs.