If you have a motor that pumps out extra HP yeah,,,it's not a must to have and some will say it's not worth having at all. If your just a trail rider and weekend warrior don't get it,, and on a stock bike it ain't gonna help squat....
it will always help if you are hitting the rev limiter but i would suggest a bigger front sprocket or a smaller back sprocket because al the 400s ive ridden should have more than enough power to still pull these bigger or smaller sprockets easily
i think you should buy 2 front sprockets one that has one extra tooth and one that has 2 extra teeth and try both see which one you like . i tried a 14 tooth on my 300 before i got my pipe and didnt seem to have the power on low end that it used to but with the pipe it has more than it ever did have. but a 400 should have more than enough power for a 2 tooth larger sprocket