Its not just about boost. MBC can add boost, but if the ECU is confused by the extra boost, it can pull timing, bleed it off, etc.

Whilst boost is an important aspect of our cars, its not everything... The difference from 17-21 psi may only be a few KW, but I prefer to look at the Torque curve than anything else...

Seems pretty obvisous by now that Giac'd Polo's are pulling slightly better numbers than APR's...

Thunder, you are likely getting more peak PSI, when I was using an MBC I found that boost came on quick, torque was hard, but it also died lower in the rev range. Ultimartely, if you are getting more peak torque but the rest of the curve is suffering, you are prefering peak torque and more noise over a more consistant curve.

when I was logging a lot, I found the best results came from timing and bypassing the N249, the MBC didn't do it for me. Nor did the J vesion of the N75. As Fuzion has said, what works for one tune or vehcile may not work the same on the next.

If you are chasing the last few KW with the existing tune you can also try:
- changing the intake (eg POD)
- removing grills on the MAF
- phelonic spacer (more about heat)
- porting / polishing the exhaust manifold
- better flowing intake mani
- decat / gutted cat / sports cat
- cams / block work
- injectors / fuel reg / fuel pump
- full exhaust
- cranking the turbo waste gate
- intercooer
- Tip / boost piping flow

At the end of the day, it would seem the stock turbo and tune make the biggest impact. K03 can do good things, but a hybrid turbo and good tune is a different ballgame without going to a garrett and a bunch of custom work.