More than likely the cpu be fine supplying the graphics card at least 50 fps at 4k. That's just being conservative. We'll know more once benchmarks are out and scenarios like this are tested.
Cpu sets the fps limit and graphics card renders the eye candy. If 5820k can manage 100fps in 2k without being overwhelmed then it'll do 4k in the same amount of frames with a faster card. Lower the resolution (1080p, probably 1440p too) = more cpu frame pre-rendering expected, so an older or less capable cpu is to struggle, reach maximum usages.
Id say your processor will be fine. At worst, a placeholder. See how it goes.
That is a very poor site and in no way is accurate.
A statement that is right on the site.
*This result is based on average CPU and GPU usage from different programs and games. It changes based on operating system, background processes activity and targeted applications. This result is not universal and changes based on differences in hardware and software enviroments. Please do not use this calculator primary as decision maker than as helping tool to understand performance correlations between different components.