Just want to point out that building a single 30 card deck costs about 600 electrum (some, like mono aether, cost closer to 1000) in my experience from tournaments. So building 12 would theoretically cost 7200 electrum. That's four upgrades + some leftover. Of course, some would probably overlap, but that's balanced by some being over 30 cards too.
And they would still only be able to farm a single FG per day. At max. With four upgrades, they would still probably be able to beat at least a few per day.
I'm not saying making unupgraded FG specific decks is a bad idea, just that I think they should be made out of cards commonly found in a normal FG grinder, plus maybe a few different cards.