Just read that thread about upgrading your Core i5-2500k or not, and apparently what can be a factor is RAM in it's performance. Right now I have an 8GB DDR3 stick @ 1600MHz and 2 x 4GB sticks @ 1333MHz.
Based on the Digital Foundry video, it sounds like a RAM upgrade would benefit fairly significantly.
https://www.amazon.com/dp/B00EUPV2RQ/?tag=neogaf0e-20
This worth it?
i believe you shouldn't really be running unmatched sticks of RAM anyway, or you don't actually get the benefit of having DDR over regular old DRAM
or at least that was the case 10 years ago, it's possible that technology has advanced since then