That is purty.
Hopefully its big enough. My car has a 40mm factory wastegate from the factory for a 3.3 L motor. Your engine is 50% larger than mine but if you are lucky a 15% larger wastegate might work. I can certainly understand why your 3Xmm didn't work.
There is science and mathmatics you could apply to this problem to determine exactly that size will work for you turbo and engine combo but you appear to prefer the difficult path of guesswork.
Good luck.
