My experiance is that they are not as impresive as one would hope, pleatly of guys throw junk yard parts together and come up with better results.
I think the real issue is those GT-40X heads need port work to even work marginaly well
and I am also not a fan of the "letter cams" that come with the motor
never heard any reliability issues, but have really been searching for them
also at 4 grand, their are better options