There is also an aero-cache at about 85,000 feet. It is a record height from the SR-71, above Honolulu airport. I guess both at there to hint at future updates. Only Mach 1 is an acheivement with a gamerscore, though. You can get the P-51 to Mach 1 in a straight dive, don't know if it'll go Mach 2, but I didn't.