Jack raises a valid point. If your aircraft doesn't have the poke then you will be at corner speed only instantaneously, if your engine can push out enough thrust it should be able to keep you there. You can go down the hill aswell to augment a weazy engine, thus sustained performance also relies upon thrust at a given altitude whereas corner speed is purely aerodynamic. That's why people (well - spotters) talk about specific excess power - the measure of stuff thats pushing you.