I remember reading it somewhere that one of the best English keepers, Shilton, was doing that and there's a story by some guy who said something along the lines that Shilton never misreads the shoot from the spot, and by the time he moved to save it the penalty taker was already celebrating the goal.
I think by following the ball you would at least put some psychological pressure on the shooter. Otherwise the shooter doesn't even have to place his shot very well, just get it on target and it's a goal because goalkeeper is flying to the Bahamas.