I would think a well fermented water kefir would be much better on whole 9. Kombucha often still has quite a bit of sugar in it. If you aren't fermenting your own I would highly recommend a few of the low-sugar/cal varities of Kevita.
I recommend fermenting your own. It's super easy, and insanely cheaper than 3 dollars a bottle for the store-bought kind.
I keep my batches going year round, and wait until they're perfectly sour before drinking them. Every time I sip a GT now I gag on the sweetness -- I can tell they are only partly fermented and still contain a /lot/ of unused sugar.
I also use decaffeinated black teas, sometimes throw a bit of honeybush in there as well. GT's is caffeinated -- and caffeine is not in the least affected by fermentation, contrary to popular thought. (Remember that chocolate is a fermented food, plenty of caffeine left in that.)
I've had this question, too. It does have some sugar in it. Would be very interested in the reply. I've debated doing a Whole30 reset. Though, I think the only think I would eliminate is the occasional dark chocolate and much more occasional red wine. I would love to drink some kombucha if I am giving up the wine.