Why would anybody trade a 18/21 wheel package for a 17/19? The only reason I can think of is a better selection of street tires. In that case, why not just have a street bike? Certainly the bike will suffer dual sporting and lose much of it's utility.
Straight ahead and faster -Bo Weaver 1970
"There I was..." -Griffin Niner Three Hotel
"One day closer to a parade..." Jonny Gomes, spring training 2013