Phil, I highly recommend you not go to a larger turbine. The .63 is a perfect match for the L28. You might get more peak power from a .82 but the spool would noticeably suffer. Like you, I used to have a highly custom internal gate. I could never get it to stay closed no matter how big my spring was. All porting the wastegate offers is to provide a larger surface area for exhaust pressure to push on the flapper door. Remember, the higher the boost the less wastegate size you need since most of the exhaust is used to move the turbine. As a fix, I made a custom divorced wastegate port that allowed a 40mm Tial external gate. All my problems were solved. The fabrication was a pain but I can almost guarantee that your boost issues will be gone. You may still have a surge problem with stock components but the boost flutter would be gone. Let me know if you want more detail. Your car should easily be over 400rwhp.
I'm curious on one other point. Are you really running a 2mm head gasket with stock pistons and a ported head? If the answer is yes and your combustion chambers are worked over you are well under the stock 7.4:1 static compression. That's too low in my opinion.
P.S. On an external gate, the rule of thumb is boost should not exceed 2.5x the spring pressure. 2x is preferred.