Originally Posted by TMonk

Time Trialist chiming in here, matching tire/rim width is fairly well known and the watt savings are significant and outweigh rolling resistance differences between comparable race tires. The Zipp study is like 10 years old and states that the rim width should be 100-105% of the tire width when mounted. Too lazy to dig it up but anyone into TT is aware of this and it's reflected in the front wheel setup.

So yes, having a bulging front tire is slow, possibly slower with a "fast" tire than a mid range tire that has the width matched.

One reason why you're not finding wide tubulars is that tubulars are starting to become an old, obsolete technology. Wider rims are a fairly modern trend.

Lastly, I'd like to state for the record that I don't really pay much attention to this on my daily riding road bike configurations, where I focus on comfort. On race wheels, whether it's road, track or TT, I do mind this sort of stuff.

EDIT: To be honest, I don't really pay attention to it on road racing wheels as well, just TT. When you spend that much time in the wind, it's really about your fitness, your position, and the sum of your marginal gains. As far as marginal gains go, things like tire/rim width and shoe covers offer a fairly high price to effect ratio