Ehh its one of those things thats been debated over the years in the car world. whether too short, or too open of an exhaust will allow too much oxygen near the exhaust valves. oxygen is the main ingredient in combustion or flammability which will theoratically burn exhaust valves. here is a debate that might help you. I would say it wont be a problem in the z's because they have a Y-pipe, long enough exhaust, and resonaters.
here is a link to some info that might help
Open exhaust & burned valves - Car Talk