I'm running 23c Continental GP4000 tires on a 25mm rim. They measure about 26.5 wide on that rim. I use the same air pressure that I was using for my older rims with 25c tires. I've had no problems, and wouldn't buy a narrower rim.
The rim is tubeless compatible, so the beads lock in pretty well. Perhaps a different rim might be a little less secure? Some other threads have mentioned difficulty when replacing tires, with a tight bead fit. But my rims are very easy when changing tires.
My only concern was that the tire sidewalls are a little more exposed to sharp objects on the road, being more spread out instead of curved back toward the rim.
On a different gravel bike with mini-V brakes, the brakes just weren't designed for rims that wide. The brake arms are supposed to be more or less vertical, not angled outward. But the rims are too wide for that to work--the adjustment spacers between the pads and the arms are too thick.
Last edited by rm -rf; 10-29-15 at 05:08 PM.