http://www.kreuzotter.de/english/espeed.htm
This online calculator allows you to input your specifications to compare different bike speeds and wattage inputs.
for example a tri-bike with a rider ouput of 300 watts will give a speed of 26.3mph on flat ground.
the same however with a stock lowracer with 300 watts will give a speed of 30.2mph on flat ground.
lowracer with tail fairing 31.7 mph
full fairing 46.3 mph
a tri bike coasting down a 14% grade will give max speed of 60.2mph with 157 lb rider
lowracer coasting down same 14% grade will give max speed of 84.8mph with 157lb rider
This is assuming a hill with no end. the only limiting factor being the air resistance.