The devs said that 8 man was SUPPOSED to be harder, but wasn't. From what we've seen, that's still the case. Your argument would be valid if the dmg incoming was in fact 1.8X, but it's not. It hits harder plus twice the number of people for a scenario more like: 2 healers for X dmg (0.5X dmg each) versus 4 healers for 3.6X dmg (1.8X * 2) for 0.9X each. The healers in 16 man are inherently more strained and therefore have a smaller margin of error. The people who claim 8 man is harder are, in my opinion, trying to justify not putting in the effort to wrangle 16 people.
This is true for every fight except one.
I have done on both 16 and 8 man. And I can tell you personally that the 8 man version is harder. The DPS really counts in this fight - getting the walker down in 8 man hard on the 3rd bomb guy requires really good DPS - nobody slacking, getting the ealker down as soon as hmanly possible.
Where as on 16 man we have to try to not kill it in the 2nd bomb for the most part.
Also on the final kephess phase the damage from the boss on 8 and 16 is the same. It's much harder for 2 healers to keep both tanks up in the beginning 40% than it is for 4 healers (only outgoing damage is to tanks during this time).
And on the second phase you can't have your tank die to that extra hit on 8 man because once he starts eating through the raid it's game over, you can't just have dps kite and kill like you can on 16 man if the tanks happen to die. The DPS race is just too tight on 8 man for that.
Think of it this way. The health on 16 man is pretty much exactly double - so to make the DPS the same you would have to bring only 8 DPS to your 16 man - which means 3 tanks 5 heals, or 2/6 or even 14 manning it.
16 man, with the 2 extra DPS makes a big difference.
16 man has a much easier time with the enrage timers on the other fights because of the 2 extra DPS classes they can bring.
While every other fight in my opinion is much harder on 16 because of incoming raid damage and extra mechanics - that one fight is harder on 8 man.