I was initially very disappointed with this software. I bought my Xcal 1 early on and many parts of the Extreme software simply did not work. I called my dealer about it and he said, "Yep, you're right.....". He agreed, for the time being, that I could give him a ring and he'd email me any new tunes that came available for my 05 GT (auto).
I haven't checked the SCT site for any later releases of the software.... Download the latest and see what you think. If your dealer will email you with a tune I seriously doubt it's worth your time/money to buy that cable.