How stiff are these actually? Pretty cheap at roughly $65 per set.

Any thoughts?

 

Adding a leaf will make your ride harsher.
What you might want to do is add air bags in the rear. This way you can control the ride height when you have weight back there. Plus it doesn't decrease the comfort.

 

Yeah, except airbags are way more expensive and time consuming to put in. Not to mention the installation of an air compressor. I would look into shackles if you want to keep the stock ride. Otherwise you'll have to replace the leafs with some others that will also be fairly expensive.