Interesting. You also changed the type of plug? You're now using an F plug, and what did you have before?

It's possible you had a plug that was too cold or too hot before, and now you have a plug that just matches your engine/fuel/prop better. The wrong plug will cause the ignition timing to be off, either too soon (too hot a plug) or too late (too cold a plug). It's like driving your car with the igition timing off, and the difference you get with a good tuneup.

(if you want absolute peak power, you can actually change plug models and use a tach to check the RPM, you'll find the plug can make quite a difference for some engines)