First use the power book the 12% reduction is necessary
Second Rightous Bow is aproximately 500 icpr assuming you fire about a shot a second in 5b 2r fleetness
4 set hytbold huntsman is apx 800 icp due to blood arrow costing no power and PS costing 199
you can now get 10 BA per minute instead of 6. so 4 PS get replaced with 4 BA
You can maxamine your power regen skills with a swapped legendary.
You can put 10% power reductions on your bow *sigh*
You can use power pots
Even with all this I can make it just over 4 minutes at full burn and I am giving up the erebor 4 set
However in a raid/group with a cappy or LM giving power I can go indefinately.
So my guess is that this is working as intended