What does the boost hold at with the silencer in? How long are the lines from the b.c. solenoid to the wastegate actuator and turbo?
As I see it your problem has to be something very simple or it's going to be a bitch to fix. Unless it's settings on your boost controller or something wrong with the boost controller/b.c. install (lines, electrical)/something wrong with wastegate actuator, it seems like you're going to have to- at the minimum- pull the turbo and port it in an attempt to get more exhaust flowing out of the wastegate side.
The very first thing I'd do is try to find a guide to setting up a Profec B spec II. Those things suck- they're finicky and sometimes hard to get dialed in. The control I'm thinking of is the gain, the boost response control- it might be that you're simply spiking to 23 and the boost would fall off and hold at a lower level, and you might be able to fix that with the gain control. Have you stayed in it at all or have you simply pegged it, seen it shoot up past 20 and let off? I cannot recommend staying in it on some unknown boost level for obvious reasons, but if you've already done it at least you'd know if it was just a spike.
All the other stuff is iffy. Slowboy should have been competent enough to port the turbo correctly to prevent creep/spike, and to make full use of the 34 mm flapper. With that flapper and porting, you should have trouble holding high boost, not trouble staying under 20 psi. If none of the stuff above works and you pull it to port it, I found a very good guide on dsmtuners that shows you exactly what to port to increase flow through the wastegate. In a nutshell you're cutting a deep channel straight to the wastegate path, taking away some of the taper of the turbo inlet in one spot on that side, and then you add the standard port on the wastegate outlet that should have been done with the flapper install, taking it as far out as possible while leaving a safety margin for sealing. I just hate to tell you to pull a turbo that shouldn't need to be ported while it could be some other problem, so check all of the other stuff first. It just seems like you've got all the bases covered- flapper, porting, tubular o2, etc. I would check everything else first.