Digital Foundry made a really cool video comparing the i7 3770K, 4790K, 6700K and the 7700K at the same clock speeds in a couple of games.
Core i7 7700K vs 6700K/ 4790K/ 3770K Gaming Benchmarks (YouTube)
In their testing all of the CPUs were able to run almost every game at 60+ fps except for Ashes of the Singularity. The newer CPUs are able to run games with higher average and minimum frame rates which offer a smoother gameplay experience if you're targeting frame rates above 60 fps.
From the i7 3770K with 2400MHz memory to the i7 7700K with 3000MHz memory gains of around 20% can be seen on the i7 7700K in CPU limited scenarios.
Faster memory can improve reduce CPU limitations, Richard touches on this subject here in
Digital Foundry's i7 7700K review.
Potentially more performance can be gained if you pair a CPU like this with faster memory, however this all relies on you being CPU limited, if you're GPU limited you'e probably not going to be seeing any gains.
TechSpot did some benchmarks with 4000MHz memory, showcasing the gains in performance that can be had in CPU limited scenarios in a couple of games, with some games gaining an additional 3-20% of performance from 3000MHz memory to 4000MHz memory.
DDR4 Memory at 4000MT/s, Does it make a difference? (TechSpot)
Should Coffee Lake feature IPC improvements that lead to single-threaded performance gains of around 10% on top of Kaby and Skylake which also manifests itself in gaming performance, I speculate you could potentially be looking at roughly a 30% gain in performance in CPU limited scenarios which are single-thread dependent with 3000MHz memory, and maybe somewhere around 30-40% with 4000MHz memory.
The additional cores are also likely to be advantageous depending on the game, as the number of games that take advantage of more than 4 cores/threads has increased over the years.
However, this all depends on the software being CPU limited on your system, because if you're you're GPU limited you're likely going to see little to no performance gains.
