Have you ever taken the EFI pipe off of the turbo? If so, sometimes the rubber insert can get folded over, and restricts air intake so at higher revs boost drops off. I used to have this problem, would hold 10psi to say 4500 revs then drop to 4. Took the pipe off, and the rubber insert was completely bent over. Fixed it, and holds 10psi perfectly now. Failing that, check for any leaks anywhere. Or its just your ct9 is running out of power in the higher rev range.