Basic rule is 25 percent max reduction flex for a given rating (although I understand AB has a 40 percent permission as they don't combine the two)
To get more, you need an artificially smaller engine by certifying a derate .. and then you can continue the reduction via flex.
Around here it looks like the 737s fly it to fence (8000 ft runway) then pull up..
That sounds like a significantly unbalanced takeoff with V1 having long gone and use of clearway. The later rotation approaching the end can give the impression of not much runway. Keep in mind that, for an unbalanced takeoff, only half the air distance to screen need be over the runway bits.
you now have the luxury of bringing up the good engine to max to get over those mountains ahead
That should never be necessary although it is optional. However, if you choose so to do .. do it slowly.