I think he got the nodes to quit flaring up. He was so hoarse after recording Full Circle, then going right into a tour. Being dyhidrated from all the sweating and not resting the vocals at all seemed to do him in that last tour. Every interview after and before shows last year, he was totally hoarse.
He doesn't have all the power and rasp back, but you are right! Great improvement. The vibe with the other guys on stage has never been better! He never interacted with the others even back in 1997 or 1999. Stapp was always so focused on singing and being in a vibe, he never really interacted on stage. I think the interaction now is really making the show that much better and making up for the 20% less vocals than he had before. 2009, he seemed at 60%. He is up to 80% in my book now.
This is a pretty good article on hoarseness.
http://ezinearticles.com/?What-Cause...ss?&id=1794312