I think it may have just been in the way you glued it all up. I don't buy the "past the performance level" stuff. My WM P-51 below has a ST G90 and i am gettting and average of 100mph out of it routinely.
I also have your Hangar 9 P-51 in the box out in the garage for this winter project. Plan on a Saito 150 up front. With modifications of common sense, it should handle it.