I've gotten MUCH faster in the past 2 years of cycling - I've gone from average 19ish mph over 40k to pretty much 24mph over 40k (about an hour) of riding on a flat course, which is fast enough to hold your own against competitive road cyclists (whom I train with).

It doesn't seem to translate to too much speed bonus on my commutes, though. It would if I hammered it, but at my typical commute speeds, there is a far greater impact by the traffic and stoplights than there are with my riding. I might gain 10-20 seconds on a big hill if I go hard on it, but that's nothing compared to the 1 minute stoplights that I hit.

I don't hammer my commutes, so it doesn't really make a difference. (Unfortunately)