I think you may be misunderstanding the GTY and upgrade process. When you book a specific category, you must realize that you may still have that category and be able to live with that. An upgrade is nice, but not necessarily going to happen, no matter how many people you have. And Disney will not let you book a category that your party will not fit in.
You would only be paying for a category 12, and anything you get as far as an upgrade will be great, but it is not a guarantee.
I always say, book what you can afford and what you feel like you could live with, anything else is just pixie dust!