You will never hit 600whp on a gt30 with an ej motor. Im at 650whp with a 6466 billet ball bearing turbo with my 07 sti on a built/darton sleeved motor reving to 8500. You will need a gt35 and race gas or gt40 to make that kind of power, and a sleeved block. Unless its a ej22t block. Then...
It all depends on your boost level. Mine started leaking at 25psi. My STi has a very similar bov on it and even when crushed it leaked at 22psi. Nothing but TiAL for me.
It could be a few things, check your spark plug gap because it could be blowing the spark out. Next check the coils you may have a weak one. Next do a boost leak test, it sounds to me that you have a boost leak and youre running extremely rich.