How far have you gotten with Mastery 6 or 7?
I got a total of five Mastery 6 coins, and already upgraded 2 champions. I got my first Mastery 7 coin today. (As soon as I logged in, because I didn't get it yesterday for some odd reason.)
I'm kind of liking the new method because it requires you to perform well instead of just picking that champion frequently. But the cost of Blue shards hurts - I'm not going to be able to upgrade another champion for at least a month.