Hi there..I found a cat converter replacement on ebay. will give 8+ HP they say.Can you give me an advice about this? My Rover 600, 97 model, 1.8 lt. has more than 100k miles on meter. My problem is, it has a less trottle response. I change my spark plugs with twin cathode split fire brand from U.S.new leads,new distributor cap,I use semi synthetic oil,new air filter,new petrol filter,It is better than before but has a power like a 1.4 engine.Is there anything I can Do to improve my car performance? thank you.