Used 10/11 gen i7s would provide good value for the price on the used market, but as everything when buying used, it depends on what the seller is asking, if everybody was selling their old rig for what it is actually worth, it would be really easy to say.
I think it rather maybe uses like 2-3 cores and switches between them, so the clock sometimes bumps higher, I have it the same with my 12400f some games dont use as much cpu so some cores boost higher, for example my all core boost is 4ghz but single core is 4.4 and during gaming i sometimes see values such as 4190 and such so that means not all cores are utilized to the max. You can download msi afterburner and set it up so it shows ingame fps utilization and temperatures that would be your best bet.
Should sell the MB and buy the correct one or the other way arround sell the cpu and buy a 3770
From what I can find (PassMark cpu comparison) its about 35.4% increase in single core performance and 72.6% increase in multicore performance.
8700k for sure, the single core seems to be the same, but 8700k has hyperthreading(12>6), 3MB more cache (12>9) and 100MHz higher turbo frequency (4.7>4.6).
Just a random question, what do you think is better for 12400f, gear 1 3700MHz CL16 or gear 2 4400Mhz CL19 stable with tighter timings? The latency is only a little bit better in gear 1 but the bandwidth is 20% higher in gear 2 for me.
Id say If you are building a new pc, then there is no reason to put money into this old one… If you by any chance want to try some overclocking on this platform just for fun to see what kind of performance is hidden, then you should buy a “k” CPU like: i5 2500k/i7 2600k or i5 3570k/i7 3770k.