Yes these were properly built cars not pieced together cars/kits. Bad tunes take out the pistons first these cars has broken rods. I've seen my fair share of broken engines from detonation as well.
I'm not saying someone can't make 550RWHP on a stock bottom end because it has been done. However it's the exception and not the rule. It seems like the guys that make mid 400RWHP power do fairly well and some still break. The ones with 400RWHP rarely break and it's usually tune related when they do. So when someone blinding comes in saying what can they handle it's usually best to be conservative and tell them the 400RWHP mark. You get someone that is completely clueless on Mustangs and say hey someone made 550RWHP so they think that's fine and they do it and break then it just screws up someones day.
The Mach 1's are pretty much the same they say stick to under 450RWHP if you want it to live for any length of time. Mine makes 468RWHP and I know I'm pushing the limits of it. Sure some have made 500+RWHP but that one envelope I'm not willing to push.
Something else to keep in mind is RPM. That is truely where I think the weakness in the rods that break is. The guys w/ 4v cars that shift at 6800-7k RPM with 450+RWHP seem to break more often. Same thing with the 2v guys if you keep the RPMs reasonable they seem to live longer with more power. There are always some exceptions.
Bill