To be honest, don't think it is worth it. If it were 10-20 years ago I could understand. You wouldn't find much people to do a conversion anymore, and you probably would be looking in excess of £1000k for a shop to do so.
Power gains now are nothing compared to most modern day cars. Not sure what engine you have, but a swap out for a 15 bhp gain and probably 0.5 second knocked off the 0-60mph isn't worth it imo.