I am nearly 200% sure that the cheap set I got on eBay (just under 30 quid) were faulty or fakes as my car ran like a bucket of shite with them.
I sincerely thought it was the car and i was just starting to notice things but next service, I ponied up and got the full service kit from NWP and the car was completely different. like....transformed.
As has been said, vital part of performance so just buy them from someone you can trust.
A