Well, not sure what you have, and what you consider standard. I'm assuming you're talking about the whole rear end ...
A stock V6er has a 7.5" rear, so the Cobra 8.8 rear is an improvement.
A stock GT is a 8.8, which is the same as a Cobra, but the GT rear brakes are smaller, so if the Cobra rear is with brakes, it is an upgrade to a GT. But the front brakes should be done as well to Cobra specs.
If the Cobra rear has gears installed, or an aftermarket diff, or better axles, then it's an improvement either way.
So IMO, if you have a GT the Cobra rear is not really something I'd pay for, unless it has mods that you want.