I've been thinking about upgrading to an FIM certified helmet. I currently have use a SNELL rated automotive helmet. Karting seems like there would be more Moto GP like head injuries than open wheel car like injuries.
Before you upgrade, make sure you are aware of the helmet requirements of the tracks and racing governing bodies you will be racing in - they may only permit SNELL. Been awhile, but back when I rode, SNELL had helmets certified for motorcycle use, and believe me, I have had the misfortune to prove the efficacy of SNELL rated helmets more than once, and if you get in a situation where the SNELL rated helmet won't protect you sufficiently, you have much bigger worries (and chance of death) than what protection a helmet gives you. Just a sad fact of crashes in life - head injuries aren't the only cause of serious injury or death.