FWIW, if you're looking for a "fast" road bike which can also handle light gravel duty, a number of the new "endurance bikes" will accommodate 30mm or 32mm relatively smooth tires. The high end carbon endurance bikes will be close in weight to a race bike but will be smoother and with more predictable (slower) steering. That said, I continue to see folks riding road bikes on gravel. It's not my preference but these riders are using 28mm tires where I prefer 40mm.
To your question, the Salsa Warbird carbon is spec'd for 44mm tires and Salsa has a reputation for being conservative with the max tire width spec. Rough translation - if you don't need clearance for mud you may be able to get by with slightly wider tires.
good info NoCoGreg, thanks. I'm with you, concerning the tire size. I ride 28mm tires on the pavement because I like bigger tires. I'm just shy of 250 lbs, so I need the extra air volume in my tires, especially on gravel. 35-38c is about the smallest I'd even consider for a mixed surface ride