Speed: All else being equal, speed is a function of motor (watt rating) and controller. Most electric bike motors are capable of higher performance characteristics than the controller allows. Motors with roughly 500 watts continuous rating are required to push a 160-lb. user down the road at 18 mph when dressed normally (no spandex) on an upright mountain bike.
[Beware: Some e-bike advertising touts their high-power motors (e.g. 600 watts), but only deliver 400 watts through the controller.]

As for power, consider that Lance Armstrong's average speed over a 2-hour ride is 20.5 mph. That's just over the legal speed of e-bikes. Lance expended about 1/2 horsepower, or 373 watts, continuously. Most e-bike motors operate continuously in the range of 300 - 600 watts. Most e-bikes, therefore, will make a "Lance Armstrong" of difference in getting you down the road and up the hill!

Regenerative braking doesn't yield much "juice" back into the battery. Even the hi-tech regen on electric automobiles gains less than 10% of the original charge. Therefore, given a choice of either regen or freewheeling, you will generaly get more range with freewheeling - unless you have a hilly route.

Due to the nature of batteries, you can double the battery life expectancy by discharging only 50% of capacity instead of 75%; you get 6 times the battery life at 30% capacity usage per cycle. Think of battery lifetime as having $1000 in the bank and withdrawing a dollar with each 30% disharge cycle -- and withdrawing $10 every time you deeply (85%) discharge the battery.