It won't change the amount you can tow unless your rear sag was outpacing your engine/transmission/differential combo. Your tow ratings are determined by a whole lot of different things including your braking ability, steering ratio, differential gearing, bearings, etc. It only adds a little payload capability- by this I mean you can load it up and it won't sit on the bump stops
I threw in the procomp AAL and it states an extra 900-1000lbs, I can safely say that it holds weight. I had over a ton in the box and it sat level.