100-maybe 200 degree drop in egts on a stock exhaust and gutted cat. You can faintly hear the turbo at idle. Overall noise is not really much difference. No big gains in mpg. Might have gotten 1mpg but that is not great enough to proclaim a gain in my book. Loosing the muffler is next on the list st some point. Knocking a hole in the ceramic is pointless. Unless your truck is running terrible is should pass a sniffer. Also unless they either run a camera or pull the cat there is no way they will be able to prove it was gutted. Worst case they fail you for opacy or something else. Visually though it is there. Proving it is not functioning or altered will be extremely difficult.