I had a problem in that I couldn't boost above 12psi.
Boost would build and then plateau towards 12psi...
Checked EBC settings in MS... no problems
Checked for boost leaks (and found a few)... helped but not much
Tried removing the external WG and using a block-off plate...nada
Was told I had a 'peashooter exhaust' so removed it...
(I saw boost spiking so I got off the gas and I'd found my issue)
I'm sure this would be obvious to a lot of people here but it took me some reading as my misunderstanding was that if my DP was 2.25" then that's the biggest restriction in the exhaust system and in fact it wasn't. The further back you go the more the exhaust gases cool and expand so a 2.3" cat-back is much more of a restriction than 2.25" pre-cat. (It's also very possible that the muffler was a restriction too.)
Reading around I found that this restriction can also cause overheating issues, knock and a few other problems.
I hope that this helps someone else with the same limited boost problem.
Anyway, the restrictive cat-back is back on the car so the neighbors don't hate me and I've ordered a BEGI 3" Turbo cat-back. Should get a controlled 18psi before long.