There are a lot of things that can cause boost creep. Anything from faulty/poorly installed/bad design boost controller, to a wastegate issue, boost leak, etc. Would need to know all your mods to narrow it down.
Sounds like that turbo isn't built well for this application. It's probably a poor wastegate design. If the wastegate hole is smaller then the ct-26 then you could try porting it out, but it's not an exact science and you could end up ruining the turbo, you'll also need to keep the wastegate flapper bigger then the hole. It's nice to be different, but sometimes different is a hard road, I suggest switching to a turbo that we know won't have the same problem.