Well they don't even ship the current Mac Pros in volume...
I ordered my new Mac Pro 8 cores 3Ghz 32Gb 1Tb SSD dual D700 2 weeks ago today and from what the UPS website tells me I should receive it tomorrow... I guess thats what you get for having a computer made in the US instead of chinaland... which make me think their costs must be much higher than the previous Mac Pros that were made in china...
That's actually pretty good, especially if you had normal ground shipping. Most Mac orders take about a week to get out when they ship in volume, so two weeks on a custom order isn't bad. It took me a few months to get a Mac Pro once because even though they were "shipping in volume" the one component I ordered custom had low stock.
I don't think volume concerns will affect a production line switchover at all. In the past they haven't. Apple seems to like not entirely matching demand any way. Makes it seem like they're shipping a lot of product. But regardless, Haswell-EP and Sandy Bridge are similar enough I don't see switching product over being a huge deal. Same basic components, same source for parts, same shipping schedule, same or extremely similar case, just a different revision.