15 secs is pulled from LVB cd's and the time it takes to get 20 stacks of fullmination. i am 100% sure that prioritizing to start reapplying the FS after 15 secs left has past is a minor dps. lets say u always cut it on 40 which means in a 5 min long fight u need to apply it 7.5 (8) times during the fight, while if u take it on 15 sec or less u only need to apply it 4.5 (5) times which saves 6 gcd's whitin those 5 minutes. which is a few extra cast.
This is theory but live ur number would be lower or even higher, while mine can be maybe 1 higher if u have this prio in the back of your head.
So how cant that be a dps increase in the long run?