I know we're talking about rear spring/sag here, but I'm really kicking myself for not installing a set of front HD coils when I did my level-struts. I do a lot of towing and have airbags which deal with rear sag & bounce, but up-front still have the stock (non-HD) front springs and the front-end can get rather bouncy while towing with heavy tongue weight. The HD models are just awesome!

 

I should have taken a picture of my 07 HD with 980lbs of batteries in the back. Settled down about 2". It takes 1000lbs for the mud flaps to touch.