I would, personally, be more inclined to believe that the level of the Qi focus is limited by the rank of the power in question, than your magic---It would be ridiculous to allow someone to take a rank in a power specifically to boost it to 6 or whatever with a Qi focus, wouldn't it? Perhaps it would be best to say you can only double, or 1.5 times the rank of the power(rounded up), regardless of Magic?
Thus, proposed Adept would get 6 or 8 maximum ranks in Mystic Armor.
That would be my personal houseruling on the matter, at least.