My experience has been that the external gates off the turbine housing just don't work very well with the new billet wheels. The problem isn't the gate or the hole size as much as the location of the exit. The exhaust gas is still partially entering the housing before it gets to the hole.
I went throught the same thing as you using the lighter spring - it would control when just rolling into the throttle but would overrun the gate when nailing it from a standstill. The lighter spring can also slow spoolup. Rule of thumb is to always run the heaviest spring you can for the best spoolup.
I'm sure you don't want to go this route but my solution was running the gate off the header and plumbing it back into the DP. Matter of fact, I used one of YOUR 3.5" dps with no wastegate provision along with some slicing, dicing, and welding with a 40mm gate. I can now control it as low as I want. PITA to fab but is the best setup for boost control going forward.