Aerospike exhaust nozzle:
As mentioned earlier, the very unsteady exhaust flow (refering to pressure, temperature, mass flow) and the highly variant athmospheric pressure and and aircraft speed makes it inpossible to find a perfect Laval nozzle for all operation point. However, there is an alternative to the Laval Nozzle and that's the beloved Aerospike. An Aerospike type nozzle can adapt to different pressure ratios much better than a Laval nozzle because of its expansion ratio adaptes to the pressure ratio.
The Aerospike design wasn't allready invented during WW2 and I've never seen it applied to an exhaust stack, but it could surly increase thrust, at least under most cicumstances. It might have been more difficult to implement on an inline engine than on a radial engine, but its intresting to figure out, how this might have looked like.
Unfortunately, with the end of the Unlimited Class at Reno, there is no usefull application of this idea anymore...