intake manifolds

it seems to be common knowledge that a dual plane will outperform a single plane on the street.
..just how much difference is there?
..i bought a '69 Dart back in 2000 that had a stock rebuild including cam with a single plane Torker on it
(which everyone says is a poor intake choice)
That 340 was quick and would squeal both 60 series rear tires for nearly a block?