So you have a cool plane, but you want to zip across the map at 10x the speed of sound?
Locate the .tweak file of the aircraft you want to mod (located objects_server.zip/vehicles/air/(theplaneyouwanttomod))
Search for these lines:
They're pretty self-explanatory but if you are completely clueless, raise the acceleration and torque in small incriments. If you overdo it then you may speed off and have no control over your aircraft.