Honestly performance ones aren't realy worth it, gives a slightly bigger spark, meaning a fraction of a BHP, in high tuned race or drag engines then you may see a difference, but for £80 its not worth it. You'll notice a bigger difference taking out and cleaning the ones you already have. If you are going to replace someone like EU will do them cheap.
I put these "iridium" tipped spark plugs in my Lotus Excel last year (at the specified service interval). They cost an arm and a leg but I thought they gave a smoother delivery of power. However, this year, not wanting to spend so much, I reverted to the standard plug. I can't say I've noticed a decrease in performance; any new plug is going to be better than the one that's done 10000 miles or so!