Don't get wrapped around the axle worrying about what RPM's you should be turning. First of all, the specs that they quote are not real world performance. So don't even try to get to what you "think" it should be doing. Second, when you put a bigger prop on you are going to lose a few RPM's. It's basic, your swinging a bigger propeller and it's going to take some away from the engine.

Just tune it to run well, and then richen it up a click or two, and you'll be fine with it.

Ken