So are you arguing that the ZL1 should NOT be compared to the GT500? in which case why are they doing 580 HP, I think more like 500 is more appropriate (in line with Boss). The GT500 is slated as the affordable supercar like the corvette.
580hp to best the existing model...and prepare for the future
I'd accept this argument EXCEPT the ZL1 is going to be TOO good and TOO expensive for this argument. In this case the ZL1 would have SS suspension, and probably a supercharged LS3 putting out 480-500 hp. Maybe make a few other tweaks like the boss has, and make it king of the camaro line, overpriced at about 46k for the camaro lovers who want the best.
The level the ZL1 is being built however is NOT to be king of the camaros. they are building a class A sports cars thats going to compete with the corvette and the GT500... That has to have effects on the chevrolet portfolio strategy.
The ZL1 is the King of Camaros. It's a very good bet that there will not be a faster factory Camaro...at least not in the 5th generation..
Let me ask you a question...Why cannot the ZL1 compete and dominate (performance) both the GT500 and the Boss? I addition to some world class cars like Jags and BMWs...
Wouldnt you agree? if the ZL1 was NOT trying to compete with the GT500 and "supercars", then it would be more like the mustang Boss... why would they be spending so much money on R&D building a true performance beast, and driving up the cost unneccessarily, lowering profits AND lowering sales OF THIS PROPOSED MARKET SEGMENT.
Why are they working this hard? To make it the best. Rest assured they aren't going to suffer financially for doing it. 
I'd argue they are doing it because this is NOT their most significant proposed market segment, as if it was, we'd have a BOSS fighter on our hands, not a GT500/corvette fighter.
Keep in mind the Camaro has only been around for 2 model years. It takes about that time to come up with a new trim level within a model. With this in mind...the Boss Mustang did not exist when they were planning the Camaro - nor will it for much longer. It's my understanding that it is a limited production/model year car.
GM stated that their job is to MAKE MONEY with the new cars, so obviously they aren't doing it to feel cool, they are doing it because they are trying to tap those additional markets.
Segmentation theory would suggest that there are buyer segments between the SS and corvette going untapped by GM. The Boss is Fords answer and the Zl1 is Chevys answer.... however, it goes against most logical marketing theory to cannibalize your own products unless it is part of a long term strategy.