Soulburn is a pretty ridiculous buff for FMs as well... I can't speak to how it effects other classes, so you might be right about KFM at the highest gears getting a bigger bonus, but there's something else to consider: FM's soulburn skill gets locked out during a knockdown or grapple (this was fixed in other regions and we should be getting that fix soon) - at which point they are gaining very little benefit from soulburn until the kd/grapple ends (as compared to the 2-3x damage from soulburn skill)