That just flat out not true. A100w multichip led is more equivalent to 150w metal halide at best.
Use a quantum counter and you will see.
hmmm.. so I actually did. I ran a 100W LED with 55 and 85 degree lenses.
This thread covers the results even though it wasn't the intent of that thread.
http://www.reefcentral.com/forums/showthread.php?t=2553733&page=12
At about a foot away, I got ~400 PAR with an 85 degree lens and 1000 with a 55 degree lens.
My 400W MHs vary between 400 and 1600 depending on whether the bulbs are old/new and whether the reflectors are wide or focused. 1600 is the hotspot for the new MHs with a tight reflector.
So, the 55 degree 100W LED is ~comparable to a 400W MH with a wide distribution reflector.
So - it depends.
But I can certainly see that a couple of 100W LEDs with the right lenses can generate as much PAR as a brand new 400W MH for some area of coverage. Or maybe one of them is equivalent to a brand new 250W MH.
The trick is in the coverage. These LED PAR reading are down the lens cone at 12" away. So the 55 degree lens covers about 120in2. That's about the size of the MH hotspot... so green apples and red apples.
The only way to truly compare is to put the LED fixture to the test on a full PAR map (just like I did with the MHs), not a spot measurement. That's in the plans, but I have too many projects running at this point.
Good point though.. it's an important datapoint for real comparison.