At 1440p ultrawide, it depends. In some games, the 1080 Ti will be the bottleneck, but you may still get CPU bottlenecks in especially older titles. The Threadripper 1950X will be more of a limiting factor than the Intel Core i7-8700K, so be sure to check benchmarks for this. Overclocking should at least help you a bit. You should still get good performance, either way.
With Threadripper, you're getting a 16 core/32 thread CPU that runs at a pretty decent clock speed, making it a (expensive) monster at multi-threaded workloads. But if your main concern is which part bottlenecks what in gaming, a HEDT CPU like this one is not for you.
With the Intel Core i7-8700K, you're getting less than half of the cores, but they're very fast cores, and you're still getting a decent amount of them. This gives you a good option for a powerful multi-threaded CPU without having to totally break the bank like what a $1000 CPU + $300+ motherboard would cost you. And yes, this is excellent for gaming, perhaps even the best, but if that was all you wanted, you could just get an Intel Core i5-8600K.
This person is getting a 1440p ultrawide monitor, though.
But yes, at 1080p, if you're actually using a 1080 Ti to play at 1080p, your CPU will be the bottleneck at least 99% of the time regardless of what CPU you choose.
