I still think that a simple port job may solve your issues.
The boost creep up top is the give away. The exhaust gases can only go through the turbine or the wastegate, if boost is climbing above target then the wastegate isn't flowing enough. The fact that the p18 is getting 1.4 bar up top indicates that its pretty well sized for the motor (unless you want more!).

You should really only be going for a p20 if your not achieving the boost you want up top. P20 and P18 at 1.4 bar is still flowing the same amount of air through the compressor. 2.5's benefit from the p20 because the p18 wont hold the boost due to the greater flow of the motor, and hence the need for more exhaust flow through the turbine.

As your blurb above states, compressor surge doesn't take into account the operation of the wastegate, and to me it seems silly to give up response without first trying to fix the wastegate problem.