Well, when you consider that in most cases the nozzles are grossly under sized for the altitude they fly from. The diameter of the nozzle exit for flights at sealevel are vastly different from the dimensions for flights from, let's say, Hartsel(sp?) Colorado. 1 nozzle on a commerical(Aerotech) turned out to be optimized for flights from Death Valley, like 300 ft below sealevel
So some kind of nozzle exit extension for proper expansion at a given altitude would work, setting it up would be the problem. With all the different perameters of all the different motor sizes and types and the almost limitless amount of different altitudes where the optimal nozzle would be required to work each and every nozzle extension would have to be made custom in a machine shop. The best compromize for exact expansion would be like shooting for an expansion that is perfect halfway through the burn, slightly oversized at the start, going through the perfect point, and finishing up slightly undersized.
Such a nozzle extension would give you a performance boost depending on how bad the design is, the worse it is the better for the nozzle performance boost.
Then it comes down to what the manufactures think, is it a mod to the motor?