Originally Posted by chessmyantidrug
I'm having a hard time following your logic. The i5-9600K can achieve higher clocks, but gaming at 1440p isn't as impacted by the CPU. If I'm upgrading the CPU, I'm adding cores and/or threads. I wouldn't consider the i5-9600K enough of an upgrade because you're only gaining clock speed, which already isn't a problem. The most logical upgrade would be an i7-8700K. There's not just some value, there's all the value. Upgrading to an i5-10600K requires a new motherboard, which I guess selling the i5-8400 and MSI Z390-A Pro would help offset, but neither of those is worth a whole hell of a lot. It doesn't make sense to swap motherboards to achieve performance you can get without swapping the motherboard. The i5-10600K is basically an i7-8700K on a different socket. Why change sockets instead of just getting an i7-8700K?
OK, there are many ways to slice this... I agree that more cores are better, but my point is that they are not used completely by everything, and games are one of those cases where the scaling is not linear (i.e. double the cores is not going to double the frame rate).
When I did a quick search, an i5-9600k showed up for $200 where I am. That's a 37% cpu performance boost if you assume it will overclock to 5GHz, and that performance upgrade does not depend on the application being able to scale to more threads.
I think 6 cores at 5Gz at 1440p will be almost completely gpu limited with a 2070 super. Or at least close enough to not matter (maybe 6 cores at 3.7GHz already is, but I was assuming not). So, for games today, this seems like the best value option. Most of the performance of any available cpu, with relatively low cost. The only downside is that it won't scale as well for future games with a possible future gpu upgrade.
6 cores, 12 threads (aka i7-8700k) is going to be a little faster in games that can take advantage of more threads in non-gpu limited cases. And more future proof. BUT, that cpu seems to be going for $300-400 on the used market right now. I completely agree an 8700k would be better if the price is right, but at current used prices I started to think about upgrading the motherboard (and selling the old one to recoup some cost). Note: I didn't do much research on prices, and so with a little patience this could well be the way to go.
So yeah, lots of options. If the OP is willing to OC, and can find a 9600k for a good price, then that will likely give 90-95% of the performance of any cpu in games at 1440p today.
Not willing to OC, then something else might be better.
Or, wait until later this year, when next gen cpus are announced. Actually this is probably the best option unless there are some specific cases today where the OP is running into cpu limits :-)