If you're benching, it makes a difference. In fact, if you're spending the sort of money this set up demands, it will make a difference. No one is going to put two Titan blacks, or two 780TIs (because that's the performance level we're talking here) on X16 X8.
So basically that particular hex core is only good for one GPU (that's why they have done it) but as we already know, when it comes to pretty much everything a 4790k is enough to let one of those sort of GPUs stretch its legs.
Making the 5820k, like locked SBE IBE Haswell CPUs pointless, and, rail roading the customer into buying the £500 CPU.
I used to work as a manager in a call centre. As such we used to have the so called "Pro sellers" come in to train our staff. They would basically draw this map on a board, and, show you that basically the cheaper products were only there as a get out of jail if your sales tactics failed. But, this map they drew would basically be the rep railroading the caller into buying the most expensive product.
And it works too. Who is going to pay the premium for a board and ram, then settle for a derped CPU? "Meh, whilst I'm at it I may as well buy the more expensive CPU....."
Human nature. And the sharks? they know it.
