I'm aware of the "water changes are the single biggest factor in the fish's growth" theory, but every organism needs energy to function and grow, and food provides that energy, there is no way around that. No matter how much water you change daily, if your fish is "burning" more energy than it consumes every day, it won't grow, at least not as much as it potentially could. In the wild, when water quality is not generally an issue, the fish that finds the most food is the one that will develop the most, breed the most and manage to raise the healthiest fry. Poor water quality in home aquariums negatively affects growth but ultimately it's food that makes all the difference... nothing survives on good water quality alone. So yes, a good, plentiful diet is just as important as good water quality.
The way I see it, powerfeeding is especially useful with young fish (I have tried it a few times in the past) and it involves feeding your fish small, varied high quality meals several times a day while keeping an eye on water quality. The difference is that overfeeding is normally related to bad fish keeping, poor water quality and sick fish, and powerfeeding is a heavy feeding schedule that you're completely aware of, so you can keep records and take the necessary measures to make sure your water quality isn't affected. Fat unhealthy fish can be avoided by providing large tanks with lots of room for swimming, digging and a strong current for them to "exercise" on. I don't recommend this practice (I realized there's no need to have your fish grow extremely fast purely for your own enjoyment) but I wouldn't go as far as condemning it or discrediting people that want to try it.
