It does have an impact. But it's less of an impact than most people think, and in a different way than you think.
A 350fps gun using .25s, and a 400fps gun using .30s both have a relative velocity of 320-330fps. The BB doesn't actually get to the target faster, it just goes further and is less affected by wind and foliage.
Proper compression of your mechbox, your hop rubber, and the consistency (not diameter) of your barrel affects performance more than your FPS alone