I found that the easiest way to improve throttle response was to install a K&N drop-in filter. Any hesitation that I had immediately disappeared, and I had a couple of friends try it in their Foci when they had similar complaints. You won't have any hp increase, but the throttle becomes much more crisp.
FocusFest 2013: Best of Show
Gateway FFW: Show Class Outstanding
September '13 ROTM