Well, I personally don't care too much about who's the fastest: so dispute my statistic if you will, but I think the more important point is that he is going to be one of the fastest footballers around, and probably a good deal faster than anybody he'll come up against in the WC (if he plays at all).
Robert Pires, among others, have released the information from the club that Walcott beats Henry in many of the training sprints. Again, this could be disupted to no end, but there's no dobut that he is very very fast, notably more than others.
Atreides: that's like saying pragmatic / well-defended teams render fast players useless. This is not true, or we'd see 0-0 games every day. It's impossible to cover Walcott, or Henry, or Martins, or whoever perfectly, or even very well.