with a full exhaust you will also see a different increase in boost spool rate that is different when you dont have exhuast. have you ever noticed that with no exhaust or open D/P you peg your desired boost rather quickly but it really doesnt feel like much is going on??? this happens when i open my cutout, it reads 12 psi but doesnt feel like shit till i actually hit like 4500-5000rpms, im pretty sure its the same as this issue

what turbo are you running?