I've become a believer in riding on the widest possible tire for the application. After many years riding very skinny tires, I decided to try 25 mm road tires a few years ago after fixing one too many flat tires at the side of the road. The reduction in pinch flats since the change was quite dramatic, so when I bought my new frame from Velobuild, I hoped it would allow me to go one step further to 28 mm tires, given that their model VB-R-027 was a bit more relaxed in its geometry than some of their other frames.
I can now report that 28 mm tires work fine, as long as they aren't really 28 mm. I got a new pair of Continental Grand Prix 4 Season tires this winter in the 700x28C size.
The problem is, I don't think they are actually 28 mm wide. They seem to be about the same size as the old tires I removed, a less expensive pair of tires that were marked 700x26C. A relatively rough measurement using a ruler and eyeballs seems to confirm that they are about 26 mm wide. (This also appears to be confirmed on various forums I've read).
Here is what the Continental tires look like in the fork:
There are maybe 2-3 mm to spare all the way around. Clearance is similar behind the bottom bracket:
The worst clearance on the frame is the rear brake bridge, which in my opinion should have been 1-2 mm higher.
I forgot to photograph the seat tube clearance, but it's about 2-3 mm. The 28 mm tires that are really 26 wide work just fine on this frame. A true 28 mm tire might work, but it would be very, very close. For this reason, I'm perfectly OK with the tires being slightly smaller than advertised.
I have occasionally had problems getting the rear wheel in and out of the frame. The angle of the dropout combined with the size of the nut on the very old quick release skewer I'm using forces the wheel forward enough that the tire rubs a bit on the seat tube and chainstay bridge. I compared it to another carbon frame made by Giant and discovered that its dropout is angled differently and avoids this problem. This isn't a very big deal for me, but if someone were worried about quick wheel changes in a race, it could be a problem.