It seems that the only thing the ZL1 wins is magazine reviews. Consider the aftermarket, however.....owners of these vehicles often modify them......not very difficult to have these cars get +700 RWHP. I believe the GT500 has that very option straight from the factory.
The European vehicles may have nicer interiors and materials with a bit more technology, but you are going to be paying about $50,000 more at least for a slightly faster and slightly nicer(depending on personal preferance) for them.
Again, different markets....People in the market for a 393 SRT8, GT500, ZL1, etc..are not going to be cross-shopping them with an AMG, M3, M5, RS6, etc and vice versa.