No, you're right, the Sierra is a poor example of a hybrid. Too little too late, as usual from GM.
My comment was directed toward the 'only 25% increase in mileage from diesel hybrids' comment. Even the Prius only gets about 20% better mileage than the Echo, the closest configured Toyota. Of course, the Prius also has all the enhancements like tires, weight reductions, exhaust treatments and all the other things that aren't related to the hybrid drivetrain.
The Insight was designed with mileage in mind, not emissions (like the Prius) and there really isn't any other car that is similarly configured, so its not as easy to compare, but it might be exceeding the 25% better mileage than its non-existant non-hybrid counterpart. However, if a 'regular car' can get a 25% increase in mileage through the use of a strong hybrid drivetrain, then that actually compares quite favorably to existing gas hybrids, and I don't see why 'only 25%' is a bad thing.
Anyway, not knocking gas hybrids, just trying to defend my comment.
However, GMC rarely makes a good example of anything.
peace,
sam