if you get HFCs you'll pass an MOT according to many on this forum, who have put HFCs on their zeds, and had MOTs with successful results. Will you get more power? NO, if you haven't gone through the process of a REVUP re-map. What most do, is put on a plenum spacer, HFCs, new exhaust system, Velocity stacker in the cold air box with a new filter. With these modifications combined and a REVUP re-map, you can get significant gains for an N/A engine. Some people get an increase of 15hp, if you're super lucky you'll get 25-30hp. Yes the car sounds different with HFCs i.e. louder. I have done all of the above except the re-map. The Cobra exhuast sounds fantastic iMO, especially with the HFCs. Hope this helps.