KIAS at altitude WILL be different than that at sea level.  while you might be able to reach 440 KIAS @ sea level in a much denser air, if you are at say 32,000 feet, you may only be able to reach 375 KIAS....... BUT!!!!!! BOTH will carry roughly 500 mph ground speed.

Besides, anything above 18,000 feet is NOT measured in KIAS, or MPH.  It is measured in MACH.  ~.89 mach (just a little under the speed of sound which is 1.00 Mach)


But if you really want to go messing around, yes it can be changed in FS.

From the default A321::

[Reference Speeds]
flaps_up_stall_speed    = 142.0                 //Knots True (KTAS)
full_flaps_stall_speed  = 113.0                 //Knots True (KTAS)
cruise_speed            = 477.0                 //Knots True (KTAS)
max_mach                = 0.82
max_indicated_speed     = 340                   //Red line (KIAS)


Change the cruise_speed, max_mach, and Max_indicated_speed to achieve what you want.

Yes this catches everyone out, in the beginning. But as posted. it is still more or less flying at the correct speed, its just not indicated correctly. The air speed is measured using differential air pressure, the problem is the air gets thinner as the plane climbs so the speed appears to fall but in reality it is not.The air speed indicator is displaying just that, the apparent speed through the air not true ground speed which a gps will show. A plane may be indicating 150 kts, but with a 100 kt tail wind it will be doing 250 kts, it just wont show it but a gps will, as a gps shows ground speed. Of course the there is the rediculous where a plane is doing 150 kts into a 150 kt wind,to a person on the ground. the plane will be hovering and going no where. the gps will indicate 0 ground speed. But the plane doesnt care it still has air flowing over its wings.