Just to chime in with my 2 cents. I have a 700c Road bike. What is currently limiting me on the Swift is the upper gearing and tire selection. Granted there is a slight (3% according to Xootr) rolling resistance penalty to smaller wheels, but what I really need is a set of 120psi tires and an approx. 110-120 top gear inch to better compare to my road bike. 95% of the time I'd say I average the same speed as my road bike.

I've actually setup the bar position to be more aero since the height is limited and I'm at the upper end of the height limits of the frame. Creating the chart has been helpful because I have done some additional calculations and come to the conclusion that a sub 20lb bike would be very difficult given the parts. This has helped me from a planning perspective. The thought was to have a bike as light as possible. The upgrades I have made have put the bike at the same weight as my Giant road bike (alu frame).

My final upgrades will attempt to push the bike closer to the road bike so I can either sell it or pass it along somebody else. I am already putting in 85% of my rides on the swift.

Really, the easiest way to go sub 20 is to go the james_swift route -- fixed gear, no rear brake, and minimal handlebars.