First; why not use the proper sized wheel ?
Second; is too wide a wheel will 'cup' the tread so that only the outside edges actually do the work. The opposite is true for too narrow a wheel, the CENTER of the tread does all the work. You'll not be getting the FULL contact patch that would be the most effective AND uneven / premature wear.
Also, too wide a wheel (without bead locks) you have a greater chance of sidewall collapse under launch and wall contact after collapse.
Tires are the greatest cause of wrecks in racing. They are the 'weak link', and you want that link the strongest possible.
SAFETY FIRST