The only gravel tires I've ridden at 700x40c Clement MSOs and 700x32 Kenda Small Block 8s. The latter scared the living daylights out of me on pavement, but were fine on the crush limestone gravel fire roads around here.

So I can't give you any insight as to 32c MSOs vs 40c MSOs, I'm afraid, all I can say is that they roll surprisingly well on pavement. As fast as my 700x23c Michelin Pro4s? Nope, but it doesn't feel like I'm riding through molasses or anything, and the ability to jump of the pavement and onto singletrack/gravel is great.