That's my conflict exactly - not knowing ahead of time! If we don't get the discount, the deluxe dining plan is a competitive cost option that I would probably go ahead and do, and pre-pay with Disney bucks. But if we do get it, the DxDP might not save much if any money, and I'd use the Disney bucks to pay on site. Not knowing is a bit of a pain.
I emailed MS earlier today to ask. I'll come back and post when I get a response.