Permitted rim widths isn't same as 'optimum' rim width. The fitment guide usually shows this along with the 'fitted' dimensions and revs per kilometre or whatever. As well as the extra drag and rolling resistance mentioned by Peter, the gearing could be affected by larger tyres, and if the tyre is much wider than the optimum for the rim, you'll likely not get to use all the extra tread as it won't be flat to the tarmac as intended.....
I would guess there is a reason the majority of competitors use particular sizes- it could be that experience has proved them to give the best results!
