I have not noticed any power loss, I drive 400+ miles on the highway every weekend as I go to school 200 miles south of where I actually live. Yes, they are loud. Even new, but I would not hesitate to put them on again. I will never go back to a A/T after my experience. That is the main reason I changed.
I'm sure 20K miles down the road they will be much louder, but offroad stability and performance, I cannot think of a tire off the top of my head that looks/performs better.
I will let you guys know how well they do offroad in a few weeks when I get my first wheeling trip in on them. Hopefully they don't fall apart like my A/T's