Dave,
Sorry to be kind of negative here, but in my opinion, your quest for a speed record with rotorcraft has no real need in this economy. What is needed is a low cost, quiet and crashworthy helicopter for training and personal use.
As for symmetry, think of a car with a flat tire on the left front. It goes thump,thump, thump and pulls to the left (just like a helicopter). If you let the air out of the right side tire that will make it symmetrical and cure the pull to one side but the heavy thump, thump, thump remains. What good is symmetry?
I think absolutely rigid blades as you propose would vibrate even though it appears to be symmetrical, just my opinion.