I think your CPU would only be a bottleneck if you are gaming at a higher resolution. I just upgraded last weekend from a 2600k @ 4.5, 16GB DDR3, and a GTX 980 ti, to a 6700k @ 4.5, 16GB DDR4 @ 3000Mhz, and the same video card. Did I see an improvement? I did see an improvement but not much. I game at 344x1440 so the extra horsepower I'm getting with an upgraded CPU and memory is serving me well when gaming. I didn't do it with the mindset though that I was going to have huge gains. Instead I upgraded for the features that are offered, that I won't have with a 5 year old chipset.
Stick with what you got if you are gaming at 1080p. If you plan on bumping up to 1440p in the future then go for the upgrade. I would stick with Z170 if all you are doing is gaming. X99 is great but its is catered to those who want to do much more then just play games. The 6700k will outperform the 6800k in games that are not CPU intensive.