i think the big question is the actual size of the plane. most times the exhaust is not the problem. In the big A-4 and F-16, both massive 17-20 pound turbine kits, they still dont have a big tail pipe, and yet we even went smaller, at closer to 3 1/2 inches for tail pipe. but the kicker is inlet size. EVF and most 5 inch fan planes are not really 120mm planes, there big 90mm planes. thrust it self is rarely the problem since velocity is more important in flying a plane well. And you can even have too much thrust, then you have to choke down the tail pipe so much you are sacrificing take off and loading up amps in the plane, cause choke the tail you raise the load on motor, which is fine if you take it into account when setting up power sytem.

Barry