Lucio had a CES exhaust on his Sportivo. Actually made less power on a dyno for him in Victoria.
NOW IN SAYING THAT, YES I AM VERY AWARE THAT THE CES HAS MADE PROVEN GAINS SO NO ONE START ANYTHING!!!!!!
Northy and Silva and the rest are the perfect example that the CES does make good gains. in my opinion, i think the stock exhaust is pretty dam good how it is, there are not many gains to be made apart from in the muffler area, but then again any changes there make your car go from sounding awesome in lift to needing ear muffs :P
In my opinion, the best Bang for your Buck mod is the cold air intake, there are numerous brands, all which make similar, decent power. i myself have an AEM intake, mainly because when i went looking i found a good price on one so i went for it. there is a much of a muchness between cold air intake brands.
Headers im not so sure about, there is a little bit of a gain to be made, mainly by the coating on the headers.
Basically, what im trying to say is, if u really wanna make more power, get out your check book and dont put it away for a while. apart from the gearshift issues and all that, its a pretty dam good car for a stocker.
Stock air box is very restrictive down low on the rev range, and when you hit lift, the bottom of the airbox actually opens to force more air into the engine, creating the "Hard Hit" into lift with the stock box. With the CAI its much smoother because there is a bit more power being made right where the lift change is. A stock rolla sportivo actually looses power right before the lift change, trust me i've seen it on many a dyno. Only way i've seen to overcome this was on a RunX rally car from england, had a programmable ECU (not sure what brand) but the A/F ratio almost doubled at the lift point, which explained the massive puff of smoke that shot out as the car went into lift :P looked awesome