Since you have an H61 motherboard, you can't overclock anyways. So buying a K series unlocked CPU won't really do anything extra for you. You can only overclock a little by changing the bus clock.
You can buy a XEON 1270v2 for cheaper, and it has slightly higher clocks and improved IPC over the 2600k. The 1270v2 has a higher 9400 passmark score, vs the 2600k's 8400 passmark score.