The Y400 is a 14 incher which i think is just right, not too big as a 15.6" and not too small like a 13".
The Y500 and 400 both look very similar except that i think the lid for the 500 is a type of metal and the 400 has a carbonfiber type mesh on it. Other than that the 500 and 400 are practically identical. Except the 400 has no numpad while the Y500 does. However, that is why im getting the Y400 is because it lacks the numpad which i hate
The commercial was just a commercial. For $800 if it came with the SLI already there that would be a total steal, many people would be all over that deal. But still for $800 you get a great machine.
I wouldnt worry too much about the Y580 having a better graphics chip. The GTX 660m is only 10% faster which isnt that much. You can overclock the 650m to match the 660m if it matters that much to you.
It does have lower battery power because from my understanding it does not have the optimus technology that switches from dedicated to integrated graphics when youre doing less intensive tasks. I would imagine you can somehow enable the ability to switch to Intel HD4000 with the use of software but you would probably have to do it manually each time, although for 1-2 hours of extra battery life i would do it and not complain xD
EDIT: One other thing to mention is you are in the market for a gaming laptop and you cant expect the battery life to be spectacular because the hardware is powerful and in order for it to give you good performance it requires a power source. A battery can sustain it for so long until you have to plug it into the wall. I dont know what you plan to do with it but if you are getting a 15.6" i dont think you are planning to lug that around with you everywhere. If you are looking for portability i would consider some other options.