Isn't Teflon something on the order of 0.01 times as slippery as engine oil? Putting that in engine oil would be like swapping Vaseline for sandpaper. I can't imagine it'd be much better in a transmission.
Besides, AFAIK, transmission fluid doesn't really have a single slipperiness per se; it's supposed to have different slipperiness in gear teeth vs. clutch packs vs. bearings, etc., so it's more appropriate to talk about friction characteristics than slipperiness.