Well it's probably a combo of having to lower your wastegate duty cycles and raising the boost limiter on the ecu (if you're not getting to target boost, if you're exceeding that then it's just some wgdc tuning). What is the target boost for the map and what are you seeing when it cuts off?
The dp basically doesn't have as much backpressure that the turbo needs the wastegate to control boost, so it just keeps going and then you hit boost cut and the car shuts down the fuel, etc. When I first put my cobb on before I could get tuned I hit boost cut in 1st gear after getting on it... I've done a lot to get the best spool, so 3rd gear makes sense for you. Even after tuning started we hit boost cut a couple times until we raised it, it was overlooked at first. It's not detrimental to the car, it actually preserves the engine by cutting power. It just feels hella bad when it happens lol.
It should be a relatively simple tune, and you'll get more out of it than the standard cobb map anyways, so it's probably worth it. If you're going to portland you should go to Surgeline. I don't know what they're backlog is but Tim Bailey is pretty much the best Subaru tuner out there.