Most flat bar commuter bikes have a very upright stem. By simply changing this for one of a lower angle (cost about a tenner) and using faster tyres you'll get practically all the performance of a road bike in the urban environment at least. You'll always loose out on the open road.
I bought a Marin Sausalito in 1997 complete with horrid, clucky front suspension. I converted to a simple, rigid fork, a lower stem and good 28c tyres - it averages only 2-3kph lower than my road bike, even with a rack and mudguards fitted. In fact it's actually better in traffic as a road bike has a 52t chainset which is uselessly high and the Marin a perfect 48t.
I agree with the poster above unless you are training or want to ride in clubs or compete, a pure road/race bike is a poor choice as an only bike because for the loss of only a tiny amount of real world performance, an audax or tourer is far, far more useful.