The problem is even with that TINY turbo hotside it is easier for the exhaust gasses to flow out the turbo and overspeed the turbo than it is for the exhaust gasses to flow out the wastegate with the horrid 90 deg bend placed at a point of high exhaust gas velocity.

If you just cut off the wastegate runner and weld it back on inline with flow and port the wastegate entry nicely it should hold whatever boost you want.

I had this same problem when I put a 60mm wastegate on a stock hybrid- still had boost creep. I had to port the manifold/turbo exhaust housing so that flow favored the (larger) wastegate runners over the turbo scroll runnesrs. That fixed it.

I don't care what anyone says- that is a bad exhaust manifold design (like almost every single one I have seen) no matter what turbo you are running.

Even if you are running a large turbo the wastetage will still open and the restriction to wastegate flow will manifest as increased exhaust manifold pressure...

With most 400RWHP rotaries and below ANY TIME you are making power the wastegate is open so you should plan exhaust flow around this fact, not design your exhaust to flow well when your wastegate is shut.