I built and flew a D-powered Thunder Roc a couple of years back. The only thing I used as a guideline was how the Mean Machines performed that I had seen fly. I'd seen several of them at club launches and none of them seemed to have even slight problems clearing the rod. As for the E flight, everything I've ever built to fly on an E has left the rod with plenty to spare. None have been as long as the Thunder Roc, but I've flown an upscaled Satellite Interceptor that was decidedly bulkier. I don't think the lack of initial punch is going to be much of an issue.
As for flying it on D motors after flying it on E's, do you really think you're going to come down the ladder? This isn't one for the local soccer field.