So i get faster and so its more effiecent to fly higher without having to increase the IAS?
Only at the levels at which your engine and propellor (or turbo jet) are efficiently producing the thrust to keep the IAS. Eventually the limit will become, as suggested above, the maximum operating mach number as the local speed of sound (LSS) reduces and true airspeed (TAS) increases, so increasing mach number (ratio of TAS : LSS).