I had Success with 2600k, mavericks imac
I also installed a 2600k in my imac 27 2011. It is running OS X Mavericks and had kernel panic etc. on startup. There was one way to start the imac (it works only sometimes):
Turn off by holding the power button,
trun on by pushing the power button once,
do a nvram/pram reset by holding the keys cmd, option(alt),p,r until you hear the startup sound a second time,
immediatly hold the shift key to start up in safe mode.
after that the imac sometimes did boot into maverics.
when you are in maverics (try it multiple times):
I had refit installed on my mac, but i think this is not neccessary.
download multibeast (6.3) for mavericks (tonymacx86.com). under drivers select system, patchedAppleIntelCPUPowermanagement, os x 10.9.0.
select build and install on your primary hard drive/ssd.
After that my mac boots up normally.
i hope it is useful for you...