I do wonder about SIBO too...but when I look at the symptoms online I don't seem to have any. The first two stool tests I had done were through doctor's data and the one I sent away today was the same one you did from Metametrix. There's a breath test that you can get done to check if you have small intestinal bacterial overgrowth...I'll look into that other metametrix test.
I forgot to mention that eating a lot of raw foods can cause a white tongue as can some supplements (at least from what I read).
I will say when I did a gallbladder flush I did feel better but the white tongue never completely went away. I've had more people mention to me lately they think it's the liver's inability to detoxify efficiently...and if that's the case I'm curious how I can help it detox easier. I know exercise, infared saunas, and epsom baths help. I would think if there was a bacterial overgrowth that a sauna and/or hyberbaric oxygen treatment may help b/c the heat and/or oxygen kill bacteria.
Oh well I'll keep searching. I'll let you know what I find out from my stool sample when I get it back again. Oh one more thing...i did do the Donna Gates Body Ecology Diet for about 3 months too and I did feel great with the fermented foods I made..but the tongue stayed the same..it got a little better but it never completely went away.