The 12 weeks is because of the Enclave Backlog. There are 6K outstanding orders, which must be filled to get cars out to dealers. Custom orders will be fit in as time permits, but no longer than 12 weeks. This allows the factory to manage production.
You get a better deal now, on custom orders, because your order does not stand against the dealer allocation. The dealers will be holding thier allocated vehicles for full MSRP, as they get a limited number until production ramps up, and the pipe fills. Custom orders stand against the dealer volume for the sales kickback, but do not affect thier delivery of allocated vehicles, so they will take these orders at any price above invoice (to make free money). They know there is no guarantee you will come back to thier dealership and buy an allocated vehicle. The dealer vehicles are usually fully loaded, as they make the most off these.
You get a better deal now, on custom orders, because your order does not stand against the dealer allocation. The dealers will be holding thier allocated vehicles for full MSRP, as they get a limited number until production ramps up, and the pipe fills. Custom orders stand against the dealer volume for the sales kickback, but do not affect thier delivery of allocated vehicles, so they will take these orders at any price above invoice (to make free money). They know there is no guarantee you will come back to thier dealership and buy an allocated vehicle. The dealer vehicles are usually fully loaded, as they make the most off these.