Don't hold your breath.
The "part is on back order" story is the worst bs story that dealers have to offer. I've gone down this exact road and was fed the same story about pulling parts from the assembly line. It sounded fishy so I dug a little deeper and a GM rep (customer service) explained that they cannot just pull parts from the assembly line. This would cause a butterfly effect on production and grind the process to a halt.
To sum it up, my back ordered part took 6 weeks to come in and the delivery date was a continuous moving target. I hope you have better luck.