More haven't had success because they are using Hi-ports instead of E-heads.... Duh!!!
I can think of 3 that have time slips showing 1000hp (maybe there are a couple more?) If it's that easy in the 25yrs'ish that the heads have been available why haven't a lot more people/shops done it?
There are probably 3-4x more e-heads vs HP heads out there.