I agree with SouthFLpix with the 2-3mph estimate. On my flat bar hybrid, I can average 17.5mph over 11 miles. On my road bike (weighs 8lb less), I can average just over 20mph over the same 11 mile route.
Comfort wise, if you get a good fit, the road bike should be as comfortable as the hybrid. Having more hand positions is nice (with the drop bars), and getting in the drops on downhills is always fun too.
If your commute is in a city with lots of lights, cars, pedestrians, I'd probably opt for a flat bar - because the geometry/bars allow you a little more control over the bike, especially at lower speeds (some may disagree). If your commute has very few obstacles, and allows you to reach good speeds, I'd go road bike.
If you're unsure - go out and ride both of them and see what you prefer.