At a steady pace at a level road, the majority of the power output by a racer is wind resistance. And not covered by your formula.

That the Sigma could get within 5% for a mountain climb isn't so hard to understand if you look a bit closer at the real problem. That you can have 5-10W of wind resistance, 5-10W of bike/tyre resistance and 200W for accumulating potential energy.

But what about the steady state for a single rider on a flat road, where you can have 300W of wind resistance and say 20-30W of bike/tyre resistance?

Mass has zero effect on wind resistance. Only a minor effect on the losses in the tyres. Front area, Cw value and speed is what defines the wind resistance. Which is why a tempo bike can have ridiculous prices.

http://en.wikipedia.org/wiki/Drag_%28physics%29