I have i7 2600 that can do x264 1920x1080 30fps VeryFast at 8000 kbps using x264 with outdated C027 capture card, with 90% CPU usage while very up-close to a high motion waterfall in a PS4 game, almost overloading my CPU.
Looking at benchmarks...
http://cpu.userbenchmark.com/Compare/Intel-Core-i7-4770K-vs-Intel-Core-i7-2600/1537vs620
https://www.cpubenchmark.net/ (use ctrl+F to search a page that shows list of high end CPU)
i7 4770k is only about 1.23 times faster then mine, I am not sure if it could reach 1600x900 60fps x264 VeryFast without dropping/duplicating a frame.
Yet, 1600x900 60fps need more pixels per second then 1920x1080 30fps, 1.3889 times more.
Multiplying width, height, frames per second as an idea on how powerful CPU is needed when using x264.
1920x1080x30=62208000 pixels per second
1280x720x60 = 55296000
1600x900x60 = 86400000... my i7-2600 CPU struggle with dropped frames.