[QUOTE=ghrocketman]With a C5-3 the Mars Lander and Space Shuttle hit maybe 250', with the C6-3 it is even lower.
With the old Cox D8-3, they both hit maybe 350', making for a longer and nicer flight.

You are talking about bulkier models here, with the Honest John or Nike Zeus they would almost fly out of sight on a C6-0 and a C6-6 for the Nike Zeus while the HJ flew on a C6-6 as well putting them almost out of sight. Anything beyond that one could just as easily determine to be a 'waste' of time and money possibly even losing a model altogether. So in the end making a rocket fly 'better' on a larger engine is a generalization rather than something being tangible or specific.