According to recent tests run at hardocp using AoTS in CPU and GPU limited scenarios under DX11 and DX12, Broadwell-E processors, running on the X99 platform are under-performing compared to the 6700k under heavy loads. In other tests performed by PCworld, synthetic benchmarks (looking at multi-core processors functioning under DX12) are showing performance benefits up to 6 cores with there being about a 10% increase in performance from 4 core threaded to 6 core. However these aren't really real world scenarios so need to be taken with a pinch of salt. Still the performance leap isn't that great considering that you will probably have to pay more than 10% of your total system build moving from a Z170 platform to an X99 one.
So despite the fact that everyone is still pushing the 5820k, I am going to challenge this. I think the 6700k is currently in a good place for games and should offer some more performance benefits under DX12. I think it will be a while before game developers take advantage of the more recent multi-core processors. So for at least a few years the high end quad cores will offer performance. Additionally the Z170 platform will last you longer.
Also, not many people have really covered this but under DX12 it is quite possible that the on board GPU will be available to run alongside your discrete GPU offering more performance, which until recently has remained untapped. The 6800 and the 5820k do not allow for this as they do not include integrated GPU's. Again this somewhat depends on developer support, but considering that more people own processors with iGPU's than those that don't I would be surprised if this was not taken advantage of.
If you really need to do heavy editing I would recommend the hexa-core, otherwise I expect a quad core will do you for some time still, despite the advent of DX12.
