I have recently purchased a 2007 Silverado 5.3 with a 4 speed auto. I havn't really been satisfied with the performance and have considered a supercharger or turbo. These items are expensive and radically change  the effeciency and lifetime of an engine. I have been made aware of a new 6 speed auto transmission for these trucks. Is it available now and where can I purchase one? Any Idea how much the switchout might cost and is there any exchange value?  

The only place the GM six-speed exists right now is in the Cadillac and top level GMC Sierra Denali pickups. Those same six-speed trannys, now only mated to the all-aluminum high-tech 6.2-liter V-8, will make it into the Chevy lineup in two model years. The only restriction right now is at the suppliers plant where they make the six-speeds. They're not setup to make many more than they're producing right now, and the extra capacity won't come on line until the 2009 model lineup. That's when the Silverados will get the new trans in the light-duty lineup. Hope that helps.  

They are on the heavy duty gas trucks too.