One year half the Dallas Cowboys went down to 4% BF in the off season (including Emmitt) - as I recall they didn't try that experiment again. They felt fast at first but they didn't do all that great and ran into injuries as the season wore on.
Marshall Faulk just did something like this (drop weight) - it seems to be working, but then again Marshall is a special sort of player, with big holes to run thru (like Emmitt used to have).
I guess it's OK to "hit the hole quicker" when you have big holes to hit.