Terrible idea. Lets just nerf the crap out of pvp priest with this change.
My suggestion. PWS cost 15% of maximum mana but also receives a 20% boost. It’s not scaling great in pvp atm but making it cost a chunk of mana while boosting it’s power buffs it’s capability in pvp while keeping it a high risk choice when it comes to your mana pool so you cant just spam it in raid and has to be used thoughtfully.
Priest healing gameplay is so dull. Just spam PW:S and that is all. maybe a circle here and there. This is a big downgrade in gameplay for the priest and decision making.
Not only that, it makes healing less fun for everyone else in the raid.
Lose Lose 
I think the problem is mana. Power word shield traditionally was a very inefficient way of healing due to high cost. It was used almost only in emergency situations when you need to keep the tank alive with an instant cast.
I agree the design is so bad because absorbs hinder every other healer. there’s nothing anyone else can do and blizzard keeps making their shield stronger. with the new shoulder rune enchant, they get 10% to shields. with 6 set tier, they have 50% chance with shield to absorb and not deplete and with full sanctified gear and trinket, they will have 25% added to shield on top of that. the devs are taking the most absurdly broken healing move that stops every other healer from playing, and continue to make it infinitely stronger. Im at a loss for words, I don’t think there are any devs even glancing at healing