You should be able to get some 26.0's in there without too much effort if you spread the clamp a bit. A great trick for this is to take the bolt out, put a penny over the inside bolt hole and thread the bolt in. It will hit the penny and expand the clamp. If your stem is not threaded on the outside of the clamp, most bolts with thread in from the backside as well. Just reverse the above.
commuter turned bike mechanic turned commuter (also a Velocity USA employee, but this is my personal account)