If I were to take a Sapphire factory overclocked hd7970 and a HIS, MSI, Gigabyte, or Asus hd7970 and put them in crossfire would there be any performance hit due to different manufacturer and slightly different clock speeds?
More about :graphics card manufacturer matter crossfire
In crossfire the lowest clockspeed is taken for both cards. But you can overclock it and you will have no difference.
But if both cards are overclocked, you'll get a performance gain, right?
Yes. It should be ntoed though that the gain is subject to several factors, like the game in question, resolution etc. But yes if both cards are OCd to the same level, yes you are effectively increasing the performance of your CF setup.