As long as it's dealer installed it's covered under warranty.
Since the item is not out yet, there has been no way to do any testing so we are unsure at this time if it provides any performance gain. It will more then liekly make the exhaust note louder then the stock car.
I had the opportunity to checkout the stock exhaust system, and it is very nice for factory stock. I wouldn't imagine more than 3-4 hp with just an exhaust only upgrade.
Add an upgraded air intake, change fuel pressure, modified valve timing and ignition timing, and remap the air/fuel tables (ECU reprogram) then you will see a better degree of performance. I was able to tweak 18 more hp out of my Honda B18C1 engine by doing this while maintaining good street manners. Just adding one part is usually not the answer and can actually degrade performance if the computer doesn't accept it properly.
Don't worry about the sound of the hipo exhaust and air in sounding anything other than baddass Pontiac. Since it will be a factory released item it will have undergone proper sound testing as well as performance. Trust Pontiac on this. Have never has a Poncho yet that sounded ricer. Even my 1984 6000STE was factory tuned to give the experience of driving a German sportscar (if you closed your eyes while driving, not factory recommended)