More to do with the stock exhausts being bloody awful. Huge back pressure from the restrictive baffles leading to poor scavenging of exhaust gasses so a hot nasty mixture in the cylinders. Although the engine runs hot on the standard set up as though it was running lean, it actually seems to run rich as the ECU tries to bring the temperature down. The standard map works well with the Stage one pipes and air filter but it's the pipes that are the biggest benefit. The stage one filter may be smaller than stock but it isn't tightly shrouded so probably allows a fair bit more air in. 10 minutes with a set of tin snips on the filter cover might achieve the same result.
