Aircraft have use supercharged and turbocharged engines since the 1930's exactly for that reason--to compensate for loss of air density at altitude. In the GTs, the boost of the supercharger is being limited at sea level at full throttle and high RPM (the bypass valve is dumping excess boost), so at higher altitudes, it should still have enough boost to reach maximum power. Plus the air resistance is lower, so in theory you should be able to go even faster than at sea level. However, there are multiple factors involved, so you would need to test on a dyno or on the road to determine what altitude would cause boost not to reach maximum. Based on my experience with aircraft, I would expect that to be more than 10,000 feet, maybe much more.
