Of course I must note that my single engine performance increased by a good little margin with P-tips. I found they gave me a little extra umph for straight and level single engine flight. You can literally feel a better "pull" whereas with carbon props you could feel the "sink". The key is the slightly higher forward speed P-tips enable which helps to keep the ruddervators more effective.
As I've said before you must have good running engines to get anything extra from P-tips. In the past guys have complained that they saw no real difference in performance with their P-tips than with their original Bi-props. It's worth nothing that these were the same guys who had no idea how many hours were on their engines, when the engines were last rebuilt, when the engines last had rings, or even how to go about properly adjusting the engines.
Note: It was discovered a while back that Prince was producing some Lazair Rotax props that were not quite right (slightly over-pitched). These props only allowed the engines to turn up around 4500 rpms static which is unacceptable. A good set of P-tips should allow the engines to turn up around 5,000+/- static on standard bore engines in good condition.