Mine is very old so it might not apply directly to your situation QX6800 LGA775 65nm 2.93GHz base. I've had it to 4GHZ.
http://valid.x86.fr/bg4n0r
which is good for 2nd place at CPU-Z. It's air cooled with old Dell Pentium4 parts, and workstation fan.
http://valid.x86.fr/top-cpu/496e74656c28522920436f726528544d29322045787472656d652043505520513638303020204020322e393347487a
if you can get the K chip to run then you should be able to OC it.
Throttlestop 8.xx is free from TechPowerUp. Try it. If you can raise your Volts, and lower your Multiplier, then with a K chip you should be able to OC.
One way OEMs keep unlocked CPUs out is to limit the watts the MB supports at the chipset or BIOS level.
http://cpuboss.com/cpus/Intel-Core-i7-2600K-vs-Intel-Core-i7-2600 I don't see any difference here. Except 5GHZ OC for the K.
i see i7-2600 running in an Opti 790 here.
http://www.userbenchmark.com/UserRun/2822576
My QX6800 is runung as a Q6800 in my Dell ,which doesn't actually exist. No guarantees but it looks good to me.
I would try it but then again I'm a little bit nuts.