Has anyone looked into this? Do the tuners (Roush,etc) give any specs?

 

I havent seen any specs on this, but there are two things to consider. First is the change in rebound rate and spring firmness. If anything, this should help towing capacity by providing a firmer riding platform.
The big hit to your towing capacity is going to come from a decrease in wheelwell clearance and/or shock travel. You dont want the weight of your trailer causing you to hit the bump stops all the time.

 

Lowering significantly lowers payload, there are ways to get it back though. Such as higher spring rate leafs, and airbags.
When I lower mine, likely with the roush, I will also get more progressive Jounce bumpers.