I have seen a reciprocating saw used on a Clydesdale with HIGH hooves (literally stilts due to neglect) but never, ever, ever a chain saw Insane. I could see just one flick of the tail and getting caught. Eeeeeeh.
That being said, I trim with an angle grinder and know that if I were presented with such high hoof walls, it would take many many coarse grit grinding pads to get them down, which would mean hours. Not fair to the horse.
<>< Sorrow Looks Back. Worry Looks Around. Faith Looks Up! -- Being negative only makes a difficult journey more difficult. You may be given a cactus, but you don't have to sit on it.
Sadly I have seen someone have to do just this, sedated the horse and pulled out a chainsaw. The hoof was so hard and thick it was the only way to trim it. Scared all of us but the horse was fine and the feet were much better, once the biggest part was cut off they could fine tune with normal tools.
"No matter how well you perform there's always somebody of intelligent opinion who thinks it's lousy." - Laurence Olivier
My dead ex husband (no love lost there) used an electric sand paper grinder thing on badly grown out hooves. It worked fine. Scared me but horse got his hooves done faster. Better shape than when started.