A 2500k will not bottleneck a 270x. You can overclock but before you do, you have to make sure you have the right equipment and that you know what you are doing. What does the rest of your build look like?
Well, sad news, you can't really overclock with the H61 chipset (the chipset of your motherboard). You may be able to move the multiplier up by 4, but that's it. You would need a Z68 or Z77 board to overclock paired with an aftermarket cooler.