The internal wastegate port size is somewhere between 18-22mm and I wish I'd measured it properly when I had mine off the car for porting but it's about 20mm. (I ended up not porting it at all because the cast iron is so strong and there is almost no room to grow the port in size on a T28.) Most of (not all) the turbulence problems with an internal wastegate can be taken care of with a divorced housing resulting in more power for the same boost level. At that point, you've got a good (not great) flowing 20mm wastegate basically. The problem is, you how have a MUCH better flowing exhaust housing (which is why you make more power) because of the lack of turbulance by the turbo exit and boost will creep even worse with this divorced setup. If that 20mm wastegate is enough to keep boost under control from your setup, then you're in luck. If not, it causes creep obviously. A 400-450 WHP setup can be done with an internal gate for sure and if that's what you want to do then it makes sense. Just don't plan on running "low" boost ever.
Really an external gate should be used at that point, but I can completely understand the desire to stay internal.