Sorry, but in reality this makes not so much sense. There is no need for pressure early on in a fight, therefore you can't balance it out with damage. You use trinkets and CDs early on that you can use them a second time later without wasting them being off CD. Bosses above 50% are usually never problematic, it's always the second half of a fight that matters most. Every boss gets more difficult and complex towards the end of a fight, getting more skills, deadlier skills etc. Phase 1 or 2 out of 3, 4 or 5 phases in boss fights barely matter. And that's why this mastery makes zero sense for harder content (high mythic keys and heroic / mythic raids). Indeed an execute mastery would be way better than this, because fights get more tense the longer they go on.