ive got a 3" Trust full stainless steel catback on my GT and i got a 16.9 kW increase, ive also got custom made extractors on but not sure whether there has been a power gain or not (have not been able to get her back on the same dyno). what you must realise is that getting extractors put on dosent mean you will get extra power, however you will (or should!) notice the car being alot more responsive..i noticed it straight away from picking her up after the getting the extractors put on.
fuel wise i havent noticed a change at all, put extractors should help wit the fuel economy a bit.
hope this helps