I checked with my dealer and they told me that the tune is not available separate from the exhaust kit for the stage 1 "power kit" from GM. Since the kit ships with an "activation code" for the tune. This smells like fishy to me, they should be able to request an activation code from GM and just charge you for the time it takes to program the car.
Why isn't GM offering the computer upgrade for those of us who maybe purchased their Magnaflow kit before the power pack was available? The only change is that they require you to run 91+ fuel with the tune.
Perhaps our GM Customer Service rep could shed some light on this? Or pull some strings to make the software available separate from the exhaust (since there are many equally nice aftermarket systems out there.)