I installed the ATAK catback in a 2010 LS3 with ~1200miles over the winter. The car is not through breakin and I have only put like 2xx miles on the catback.
ATAK definitely sounds better than stock exhaust did. It is not crazy loud or anything and I am happy with it. By comparison it is worlds better than my 99 LS1 m6 with flowmaster on original pipes. I would say that this exhaust sounds as OPEN as it could, considering that there are catalytic converters on it.
Some interesting observations that I have noticed so far, keep in mind I am trying not to go over 4k RPM atm. I have noticed the NICE popping it produces at certain times. Sometimes when starting it will make a nice pop on startup. Or if I let off the gas on decel sometimes i can here it pop a little bit.
The pop does not sound crazy loud to me, but am curious as to if it will get louder or change much on higher RPM decelerations or with breakin of the engine / ATAK catback. I sort of hope that it does, I LOVE it.