The turbo should spool faster with a larger downpipe.
If your gate isn't able to allow you to run 3psi, either the routing of the exhaust is not flowing enough or the gate isn't big enough to allow you to run 3psi. Sounds like its not normally a problem, but during cold weather it becomes one.
You might have the gain set too high on the controller as well, which will allow it to overshoot boost by opening the w/g too late.
Evan
BTW, don't you have that tial 38 piped off of an internal wastegate hole? I had that setup a couple years ago and boost was all but uncontrollable. Moving it to the exhaust manifold made everything more consistent!