Where did you hit the PNZR?
I've found my best "kill zone" on the PNZR and Osty to be dead six. The M8 cannon is only 37mm, and it doesn't have the punch of the Osty's cannon, so anywhere other than rear quarter shots or dead six shots are risky as hell. Kinda BS when you consider the Osty is also just 37mm, but it fires a "hybrid" mix of AP and HE shells I guess. I find Ostys can knock out a tank a helluva lot quicker than the tank can kill them if they stand toe to toe.
Anway, the Greyhound's forte is not firepower, it is speed; use it to stalk your prey, use ground cover as much as possible (the M8 blends in better than either the Osty or the PNZR IMO), get behind the bastage and open fire. ZOOM in as you come up behind him......some drivers have taken to driving with their turret facing the rear after you pop them a few times in your lil ole M8, so be careful. If their turret is facing you, slow down to first gear, jump in the turret get off a couple shots in his ass, then run like hell. Even the MG on the PNZR turret will kill you, the AH M8 is made of cardboard armor, ain't worth a damn.