You are losing some potential in the center. One constant I noticed when toying with blade shapes was that the interior of the blade made a difference in how ultimately fast it can rotate. The closer you can get to the center the faster rpms seem to be possible. Wider blades at the extreme make a blade more sensitive to wind but the tradeoff is that as it spins faster the blades quickly stall and you reach a premature top end speed. The material is also going to have to support the weight of the blade and more material out from the center is more stress on the blade. By keeping the meat close to the center you raise the potential at the top end of the spectrum. Right now you have one sweet spot in the blade and virtually nothing to garner wind force at the origin and tips. Dont throw out what you have now, you can just move the holes out from where they are now and reduce the length from the inner diameter of the blade accordingly. And remember a sharpened leading edge is not a bad thing.
Ultimately the best blade will draw consistent force across the entire length. But when using pipe like you are doing it will pay off to keep the blade widest near the origin and taper to nothing at the tips.