Don't bother. If you're not going to change out the pistons, just shave an N42 head 0.030" and you'll be at 9.5:1.
The P90 head is good, but it really should have flat-top pistons to have any kind of advantage over the N42.
If you want a bolt on solution, that will work. But why raise the compression on a good running motor? The P90 head is a deep, modified wedge chamber. It's designed to work optimally on a flat-top piston. Running it on the dished piston in the turbo motors was done specifically to lower compression, but even running it as a flat-top motor would have been perfectly acceptable for the stock boost levels.
Bottom line, unless you're willing to change out the pistons for flat-tops, don't go to the trouble of modifying a P90 head. Cut an N42 or N47 thirty thou and you'll be at the same compression, with a LOT less time, trouble, and money.
Edited by Xnke, 22 January 2013 - 08:26 PM.