Like a few have said so far: Depends on the fish.
Big predators will more likely handle power-feeding better than fish that are habitually grazers in nature (i.e. discus etc.).
I don't mind feeding my young fish a lot so that they grow quickly. However, you only need to visit a public aquarium to see what happens when you overfeed adult fish with a fatty diet. Tail lobes get bent, and you'll see layers of fat on their head. Aquacultured fish are also susceptible to 'obesity' - I've cleaned a lot of aquacultured fish here (from 30-45cm / 12-18" in length) in Australia that had their entire gut enveloped in about 2.5cm/ 1 inch of fat.
Fish physiology is very different to mammalian, bird, or reptile physiology, and it varies between families of fish too. For one, fish use ammonia as their method of excreting waste resulting from a protein-based metabolism - not urea as mammals do, or uric crystals as birds and reptiles do.
Fish's tolerance to protein levels in their diet would also be completely dictated by what they are. Most people who keep African cichlids know that for the most part, Malawi cichlids in particular are omnivorous, and require a substantial part of their diet to be vegetable matter. Feed these things on a pure protein diet, and not only will they become unwell, but if they live, they accumulate a lot more fat (workmates witnessed a 20cm Aceii a local had been feeding barramundi pellets all its life).
Anyway back on topic, I'm not voting because the situation varies from species to species. I believe that YOUNG predators can be power-fed to a degree without too much detriment, but that care must be taken to ensure that your fish are compatible with this technique.