If your BIOS doesn't offer control of FSB, and Voltage your options are very limited.Considering how inexpensive CPUs for that are I would look into just getting a better one.
From what I see here i7-875K is the fastest CPU and it's unlocked multiplier.
http://www.userbenchmark.com/System/Intel-DH55PJ/119
Your MB may or may not support this but it can be done by other methods anyway.
https://www.techpowerup.com/forums/threads/throttlestop-overclocking-desktop-pcs.235975/
I do see the occasional XEON there. I would look into the X3470 which is the same speed as the i7-875K.