This is another common misconception broadly accepted as fact around here. The correct answer is "It depends." On lower powered ebikes it is common to limit max speed through 3 methods other than just battery voltage. One is using a jumper to activate a programmed speed limit in the controller. Another is to use an adjustable pot in the throttle line to limit throttle voltage in a tunable manner to match max speed to fit the laws of that country. The other is through current limiting, where adjusting the resistance of the shunt would increase top speed. It won't increase no-load speed, but the effect on max speed on the road can be dramatic.
Use Justin's simulator at http://ebikes.ca/simulator/
. Plug in a nice combo for the motors available from the list such as a 9C 2805 in a 20" wheel. Set the battery as the 66V Dewalt pack, and then try the 20A controller vs the 40A controller and see a 10kph difference in flat land speed.
Current limiting makes for a really nice smooth ride with no jerky throttle response, and in areas with very flat terrain I'd go as far as suggesting the combination of a high speed wind motor with a low current limit on the controller as a bullet proof system resulting in a silky smooth bike that anyone can be allowed to ride.