Well, the verdict is in. They do fit - I think that without Fenders the Felt QX could easily take at least 47C tires. With my SKS Velo 47 fenders I had to trim the fender flares a bit with a utility knife to clear the shoulder knobs.
As expected the ride on gravel became much better with the Smart Sams. Potholes don't jar nearly as much and I can take on loose gravel and sand with much better authority.
Speaking of fenders, it should not be too hard to come up with a two piece mudguard design that wouldn't restrict tire clearance at all. Do such exist already?