I thought so too, I called Keith Baker and asked, he says the old 100 aint gonna get much better than that till I get a few more gallons through it, and in defense, it is still pretty tight. I think with a Mejzlik 26x10, it would pull closer to 6200-6300, havent tried it yet. The NX seems to be a tough prop to turn, very stiff, and wide. The thrust is fine, its on a 23 - 25lb airplane, will hover at 1/4 throttle, and it rockets out of it. If I lean it much more it gets pretty hot, with no increase in RPM. I've spent a lot of time with the tach and screwdriver on this motor thinking the same as you - been doing it like the instructions say, maybe I missed something?
As for the front air dam, the cowl is pretty tight fit around the carb, certainly blocking any air that could be causing roughness over the venturi, doubt thats the problem as its smooth as silk at full throttle and idle, but what the heck I'll try it. I guess my real question is am I waiting for the diaphragm to break in or am I looking at a pressure problem, or something totally different....