I presume since putting on the exhaust the boost has gone up correct?
Does it surge up to 10 then drop to 7 or stay at 10?
If its surging to 10 and dropping back, then you' probably need a better wastegate actuator.
If its staying on 10psi, then you may possibly need a bigger wastegate.
My 2cents anyway
90 Celica GTR Turbo 184kw @ 14psi at the front wheels (Now running 17psi watch this space for dyno )
"Owner of the fastest Celica GTR in NZ (Until proved otherwise )"