the problem you are describing i've only heard after putting a downpipe and/or muffler on for boost creep. mabye this is a different problem? did you explain it better in this thread? I can always go back and read to see what's up. are you able to change the timing, boost or fuel with the system you have? if so, if you could take a log of a run, then mabye I could offer a suggestion since I know a decent amount about a utec, which in short, is very similar and the exact same thing as a utec when at wot.
I don't know if 16.5psi is what you should be seeing or not, as i'm not familiar with the cobb reflash for a sti. on a wrx, even at stage 2, the boost doesn't usually go up much past 16.5psi from the people i've talked to. in fact, it stays much closer to 16psi. i'd be really interested to see your fuel map, to say the least.
are you getting the cel's pulled? what are the codes? that would help some too.
shoot me back some info about this or let me know if I need to go back in this thread and read some, etc.