Ok I felt a digitally enhanced picture of the turbo was required for this
3N16M4 had it right, when you stick on a decent exhaust you make the main turbine be able to flow a whole lot more than the wastegate can comparitively flow, this means that the wastegate can no longer control the speed of the turbine, which means you overb
st. To rectify this you have to increase the flow of the wastegate, easiest way to do this is to bore it out.
This means increasing the diameter of the bit with the green arrows in relation to the bit with the yellow arrows, In practice you can nearly get them to be the same diameter.
My preferred tool is a carbide burr in a die grinder (or drill but it wont work as well), you can also do it with a rat tail file if you are dilligent (like 8 hours of filing) or just grindstones in a die grinder or a drill if you can find small ones, but bank on going through a couple its cast iron remember.
Ideally you want to enlarge the wastegate hole from its tiny original size to just within the outline of the flapper plate thingie. The plate has quite a bit of movement so you cant really go closer than about 1.5mm to the outline of it without risking having a small gap all the time, which will make it ever so slightly more laggy
The red circle is approximately the outline of the shinier bit of the wastegate sunken bit, this is where the flapper has rubbed it smooth, so at all time the hole must be smaller than this. The blue line is approximately how large you can make the hole, significantly bigger than original, you can also smooth out the transition into the wastegate so you can get even more flow that way.
EDIT: Just had another look in better light, the shiney bit is actually about 1mm bigger than the red circle, so you can bore it out a wee bit bigger than the blue circle, you'll easily see how big you can go when you get in there anyway.
Also Do you guys recon this is owrthy of the infobase? Its quite an important issue for anyone that sticks a big exhaust on.