axle back = low/mid gains
cat back = mid/high gains

thats why getting the right combo of parts helps with power gains. alot of times when they show dyno charts of their products you have to look at what the other parts are adding too. i wanted low end power gains since i didnt have a choice header wise (4-2-1 only avaible for accords ) so i got an axle back w/ custom piping to increase my low end power. i'd give up the high end gain to have the power off the line