If I am understanding the wording on the new GoAK correctly - it copies 10% of all healing that you do while it's up (AoE and single target) to all targets within 10 yards. Therefore, if you do 100,000 HPS with GoAK up and have the entire 25 man raid within a 10 yard range, GoAK will do 250,000 HPS (about 3.8 million healing) - in addition to the 100,000 HPS that you do to trigger it. That puts it in line with cooldowns like Ancestral Guidance. If you do more than 100k HPS with it up (which is highly likely if you stack it with AW and DF), you could probably see 6 million+.
It will still be very dependent on how stacked the raid is though. On a spread raid where it's only hitting melee, you will lose up to 2/3 of the value.