for the xfx cards, I dont know, I havent tried with those.  try setting -5mv to start and see if that sticks.  For the other question, yes it is worth it, but if you've never done it before I highly suggest only doing it on a card that has a dual bios switch (like the sapphire nitro).  there are many guides out there, in my previous comments I put a simple step solution to get it working.  but bricking cards is real, hence the dual bios switch.  and yeah its better to dual mine, but it uses much more electricity.