Steve, I think that qualification, that something is a heavy feeder, is based on people's experience with certain genera.
For isntance, the heaviest feeders I grow are Vandas, meaning that, during active growth, they can use and benefit from levels of fertilizer applied much more often than other types of plants before getting "over-fertilized" and showing tip burn. There's a hugely noticeable difference between vandas that are fed heavily in terms of how much they'll grow and bloom in a season, and those fed only lightly. Side by side, you'd be able to immediately tell which got the food. Under the light levels vandas need to bloom profusely, heavy feeding and watering definitely helps them out.
Maybe other people have experience with different kinds of plants that really benefit from pouring on the fertilizer during active growth; vandas (since I grow them) are the only ones I know of offhand.