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I have 40mm tires on both of my road-ish bikes - a Salsa Vaya and this weirdo dropbar-converted mountain bike frame.
Yeah, there's probably a significant aero hit, but whatever. I'm not racing. They ride SUPER smooth, don't weigh a ton, and are extremely confidence inspiring on steep descents - especially if the pavement isn't the best.
If I were going to buy a new road bike today, I'd consider 28mm the minimum acceptable tire and prefer clearance for at least a 32.