I've used 7x, and even 8x, but I can't say that I think it makes much difference, other than more fish lost to broken tippet.

I'm somewhat of a believer in theory that if you're getting too much drag, your leader is too short, not too fat. I fish mostly tailwaters and spring creeks ("technical" waters) but even so, I'll fish the largest leader that will fit through the eye of the hook.

I also believe that fish see even the finest leader, flourocarbon or not, just as they see the hook. They just choose to ignore it.