My 2014 FiST is having the same issue, where at WOT it will hit full boost at 21psi (cobb stage1, straight-piped with catless DP), but the bypass valve also makes that "pschew-pschew-pschew" sound. Also, the exhaust backfires multiple times in a row and it accelerates rough. Usually happens around 3k-3500rpm. It evens out around 4500rpm and is smooth from there, but only sits around 16psi. I checked all the hose clamps already and nothing is loose. I've reset everything to stock with accessport and it still has the same issue.
I'm at 34.8k miles, I'm in NY, and Im supposed to be driving home to NC tomorrow.. roughly 700 miles.. just started happening today. Not sure what to do from here other than swapping it back to stock and taking it to the dealership as soon as I get back. It's gonna be a long drive home without being able to hit boost... :/