A wider smooth tyre is faster than a narrower one in terms of rolling resistance but yes, as you say they are slower to get to speed because of the higher rotational mass. This higher rotational mass would help them hold on to the speed better than narrower tyres too.
You mention that comfort is secondary to speed - the two are in a way linked, which is another reason wider tyres work better than narrow ones. The handlebar buzz on a road bike with narrow tyres, or the jolts out of the saddle, or even the oscillations of suspension components are energy taken out of your forward momentum. Wider tyres transmit less to the rest of the bike so the performance improvement of wider tyres is more significant on less than perfect surfaces. Of course this is all for the same type of tyre - knobblies have much higher rolling resistance than slicks to start off with.
At higher speeds wind resistance plays more of a part, but it seems the wider tyres don't make much of a difference. Quoting from the Off The Beaten Path article referred to above:
"We tested both 25 and 31 mm-wide tires in the wind tunnel. The result: The raw data showed a 1% increase in wind resistance for the wider tires, but the results weren’t statistically significant. Even if we accept them at face value, the added wind resistance is too small to make a noticeable difference. For example, at a very high speed of 40 km/h, decreasing your wind resistance by 1% only adds 0.4% (or 0.14 km/h) to your speed."
Michael
