It's quite feasible given how much of these "HID watts" are turned into heat and not light. And the "600w" is what the potential is IF the led was driven at maximum power, not what they claim the equivalent is, as, for example, the Mars Eco 600 claims to replace a 250w HID whilst pulling 235w (not much of an initial saving but when you start adding on the costs of replacing bulbs every year...), the Pro II 600 replaces a 400w HID whilst pulling a similar 235 or so watts, and so on.
I mean, are you disputing that, say, an 8.5w Philips A19 led replacement bulb puts out as much light as a 60w incandescent? It's the same thing, the incandescent converts a LOT of that electrical energy into heat energy, which is no use as lighting, hence why they, the incandescent, are so inefficient they're being banned in various lands? The devil is in the detail and the BIG saving is in the conversion of electricity to heat, meaning more energy is turned into light meaning MUCH better efficiency.
The misleading part is when you see them claiming "600w" but barely being driven hard enough to out shine a candle, which is why I said previously that the "grow light" market was a minefield and, after completely confusing the crap out of myself with the numbers, I decided NOT to buy a Mars or any other "branded" grow light and stick to what I know with DIY led's.