That's a tough question - so many variables - but I'll give some thoughts.
Chasis/suspension/brakes - a non-issue really. It's just how you want to perform on the track.
Engine - probably 550-600hp before the long-term reliability is seriously compromised. And it depends on the conditions, too, right? 600hp is fine once in a while but a day turning hot laps at the track could blow the motor (connecting rods, valves, whatever).
Transmission - usually the weakest link for most cars, long term probably compromised after 550hp.
If you're around 550+, upgrading even our upgraded axles and rear end might be in order, especially if you run slicks. And definitely the transmission. After that, 550hp, maybe the motor too? Cooling system as well. Upgrade the fuel injectors, perhaps the fuel pump (can't run too lean!).
Keep in mind our 6.1 motors may be fine at 100hp per liter (610hp) but the question is, for how long and for what purpose....??
This is from experience, friends experiences, upgrades offered by the major tuners (Hennessy, Saleen, AMG, Rousch, Ligenfelter), years of results from magazine tests, etc. Hennessy could tell us more, I'm sure.