flat slide (D) carbs have a shorter passages , Less disruption of air flow in venturi gives carbs better response and power.
the round slide in the stock carbs is a little smoother and less thottle responsive in comparison , longer distance to chamber .
i like flat slides and i love air strikers the quad vent flat slides , there is never any hesitation even if the throttle is slamed wide open under rough conditions ( high speed bumps) /whoops , it wont bog out by a fuel cloged vents more vents more air .