I think the biggest downside to using a wider belt is replacing the sprocket cover. Anything wider than the belt used by QPD and there should be interference with the cover. On the QPD pulley the side plates are no thicker than 1/16" so you're not gaining much by omitting them. A wider belt would probably interfere with the shock and/or frame. You can't go anymore inboard so the thickness has to be added outboard. I'm already running without the chain guard so no biggie there.
