Sounds like BS to me, virtually all the symtoms could equally apply to a single plate as well, given the right circumstances.
We had a twin plate that was fitted to the Aussie GT Falcon in the early 70's, if that was simply installed without careful measuring of finger heights etc it was useless, fitted correctly they gave few problems, yes sure it had a few rattles etc, but so did the rest of the car, it was after all as close as you could get to a road/race car at the time- just like cobra's. If you want quiet- no rattles etc then a cobra probably isnt what your looking for.
I dont get it when people use the clutch for engine braking, isnt that what those big shiny round things behind the wheel spokes are for.
Jac Mac