for good efficiency, a small turbine engine would have to turn 100,000 rpm and really hot exhaust at 1000F ... it is not safe in a car driven by mortals

i think the limiting factor is the property of air at ground level which we cannot change ... i mean, if only the property of air is different, the required rpm and temperature could be brought down to a safe level

its 4am, i must be babbling nonsense ... let's wait for inputs from a real jet engine expert