With 6.1 being launched tomorrow, the garrison follower ilvl cap is being raised from 655 to 675. Once you raise 2 (or 3) followers beyond ilvl 660, you become eligible for the BRF cache quests, and similar to the last tier the quality of caches you receive is based on how many bosses you've killed at a given difficulty.
Since we jumped directly into heroic, unless you've been pugging a lot of normal BRF, you probably don't have the required kills to get heroic caches and would only get a 665 reward. However if you were willing to wait a bit before getting your followers to 660, you could potentially trigger a mythic cache right off the bat.
The best research I can find suggests you need 21 kills at a level to get a cache of the next level (2 clears + 1 which is the same reason HM required 15 kills). If you've been there for every guild kill, you should currently be at 12 heroic boss kills. Assuming we get at least one down tonight (13) and repeat all kills by Thursday next week (18) you will be really close to the mythic cache requirement. I am also reading that the boss kills don't need to be "lootable" kills....you can re-kill bosses and still have it count towards the total, and if true I'm just going to heroic pug gruul 2 or 3 times.
I know it's a bit convoluted, but the 30ilvl swing between normal and mythic is a big difference. I'll keep reading based on what people find out tomorrow and post back here if anything changes.