IMO, the HD650 is similar to the LCD-2 in it's frequency response alone. And it is only similar in that they both have bass impact and a complete lack of brightness, but they are not identical in FR. The LCD-2 has better, more defined treble and a complete lack of bass bloat, as well as a lack of the upper bass hump that the HD650 has. But that's where the similarities end. They not only have drivers developed by different companies, they are completely different technology. A dynamic headphone and an orthodynamic headphone having similar frequency response means very little, because there are so many differences in other areas. It's a good comparison if you are like Mr. Green and and your number one priority does involve FR- having a cold, bass-light, treble laden sound like the ER4S. In that case, you know that all the other differences don't matter to you, and you won't like the LCD-2 because it won't have the aggressive treble you prefer. But the HD650 and LCD2 sound very different in the end. It's kind of akin to saying that an apple and a banana taste the same because they are both fruit. If you hate all fruit, then you know you won't like either one, but otherwise they are both worth a try on their own.
But I think people get confused when they read these comparisons to the HD650 because they mistake frequency response for speed. They are completely different things, but people usually think bright headphones are fast and warm headphones are slow. I think what annoys most people about the HD650 is not really it's frequency response (and the LCD-2 has better treble and no bass bloat) but it's dull/slow sound, which the HD600 also shares, and which even my sister with completely untrained ears could hear in the HD600 (which has a very neutral FR- more treble and less bass than the HD650) when listening to it after the LCD2. And this dull, slow sound is nowhere to be found with the LCD-2, it is faster then any dynamic headphone out there. It's impossible to really say for sure and I haven't listened to the HD800 much, but I would guess it is faster then the HD800 (brightness =/= speed).