The internal wastegate (WG) on the stock turbo does not cope very well with higher boost levels. I have not been able to dial in a linear boost curve due to a major boost peak at around 4K rpm followed by droop after 6K.
It looks like the less restrictive exhaust and intake parts has introduced boost creep where the exhaust finds it easier to go trough the turbine instead of forcing its way trough the tiny WG.
With the stock intake and exhaust parts this is not a problem.

I have read some interesting threads on about uncontrollable boost pressure issues with separate gas down pipes like the one BEGI sells. For some reason the separated WG pipe creates an even bigger boost creep than with the stock DP elbow. The solution has been to port the WG hole or lengthening the WG rod to make the WG door open more and sooner.

Since my car is parked for the winter I decided to take a shot at enlarging the WG opening in hopes of better boost control. My goal is to have a stable 12-13 psi from before 4K and to redline.

Removing the turbo was supposed to be a 2 hour job. Instead I used two late evenings and a lot of cursing. A workshop manual would have saved me a lot of grief as it is crucial to do things in the right order to save time and be able to reach all the bolts and junk around the turbo.

Some pics during and after the disassembly. The manifold is designed for compactness, not flow.

And its off. Not the biggest turbo around. The compressor could have been a couple of sizes bigger. A deadbolt upgrade would have been fun but pointless as I plan on going for a GT307x, FM manifold and external WG next year. E85 should help squeze out som extra ponies from the stock hair dryer.

The internal WG hole is 18 mm and is covered by a 25mm wg door .

Before starting the operation I covered all ports and openings with paper and tape:

I used a coned grinding stone to enlarge it to 23 mm.

Before and after shot. (Yes my Nokia E61 camera phone sucks..):

I hope this will be sufficient for stable boost and no boost spike at 4K.

After looking at the WG mechanism I think the main problem is that the actuator does not have enough throw or the WG arm is attached too far from the flapper axle.
A spacer between the actuator fastening bolts will help on the throw but will also make it crack open sooner. I might add a couple of washers just to be sure.