For my buddies 07 MB SL550 and WOW this thing really works no more throttle lag and took maybe 1 minute to install. He has been complaining for a long time about it and having driven it they have HORRIBLE throttle delay but its GONE no more nada goodbye.
Is it the one from Performance Autoworks? That place is pretty close to me and they'll let you try it out for free supposedly...They're available for the srt-8's, but for $329, it better be damn good.
Here's a link to what they have... http://www.performanceautowerks.com/catalog/index.php?cPath=330_250_282 It's pretty decent they let you try it before you buy it.