The last time I looked into racing clinchers I came across a Lennard Zinn Q&A on the subject which showed that race quality clincher tires actually have significantly lower rolling resistance than race quality tubulars. Enough so that it would take a climb of +8% for the 1lbs lighter tubular setup had an advantage.
This is an older article so maybe some more accurate data is out there now, I'd be interested to see it if anybody has some.