We have a 3 gallon Clarke tank on our '08 DE300. The tank started life on an '05 200, but we ride mostly long loops, so when we sold the '05, we put back the stock unit and kept the big tank.
The tank is very narrow at the knees - almost as narrow as the stock tank. It gets a little wider up front, and of course, it's much taller than the stock unit, but neither of these characteristics seems to affect mobility on the bike.
Using an '05 tank with our '08 shrouds, we did have to elongate a couple of the mounting holes in the shrouds. Also, the bigger tank requires a short metal bracket for the front mount. It's a little hard to get at and needs to be loctited.
Our tank has been in service for almost six years and so far, has been trouble free except that about a year ago, the rubber gasket on the inside of the gas cap split and had to be replaced (about $5.00 plus shipping from Clarke).
