Bethel Johnson is no faster than Chris Chambers, and neither is Deion Branch...and Johnson doesn't do any more on offense than Gilmore does. I remember when Bethel was drafted in the 2nd round, everyone knew it was a reach, but there was a run on speed receivers and Bill wanted to make sure he had one speed receiver so he grabbed Bethel earlier than he should have. And since then, Bethel has not lived up to his billing on anything but special teams.
As things stand right now, Bethel Johnson is no more a key to New England's success than Wes Welker is to ours.
I would agree that Branch is fast, but I honestly think Chambers is faster. And right now Belichick's D is slower than ours, mainly because of holdovers from Wannstedt (Taylor, Moore, Bell, etc)