I'm not really worried about my GPU. It's a GTX 970 Superclocked. I'm more concerned about my CPU.

I have an i5 2500K 3.3 GHz. If I were to raise it to 4.3-4.5 GHz, would there be a mild increase in performance? Would my FPS go up? By how much? A little? A lot?

I don't think it's worth it, no.

For me I feel that overclocking made sense back in the 90s when the games often wanted more performance than most CPUs had, but these days it's kinda opposite with CPUs having way more performance than most games need. It will also make the CPU a lot hotter so you have to make sure the cooling is very good.

So I won't get any real FPS boost? Basically just 2-3 FPS boost at best?

Your CPU has a turbo boost from 3.7Ghz. So that would be an increase from 600-700MHz.

Firstly most games will always benefit from extra CPU speed instead of extra threads. Once the core is maxed out, hyper-threading won't do much.

More cores however will give you more performance, if the game supports it. !!!

For CPU bound games, the performance would be higher of course.

I wouldn't put my hand in the fire, but i thing, 10 fps gain (more or less) should be your profit here.

I don't thing you need an CPU overclock, with an GTX 970.

Too much will be always too much, even for a Titan.

For Skyrim, I wouldn't bother. That gear should give excellent performance by default already, even with massive mod load.

It's also important to remember not to exceed 60 FPS in any case.