Catalysts.
The cat on the F is surprisngly good, although a parts bin item (
what isn't?
) it appears to be designed for a greater through flow of gasses than the 1.8K can deliver - overspecified.
This is bourne out by the cat test we did a few years back, the results were inconclusive because no cat at all (best case scenario from an exhaust backflow point of view) produced only +2 on an MPi and -2bhp on a VVC. When gains are this low they are very difficult to measure accurately, the results can be explained away in a myriad of ways from changes in atmospheric pressure on the day, abient tempratures on the rolling road sessions, driver error by the tester, or even momentary loss of traction on the rollers.
Bottom line is there is no real gain on any of the cat options out there from a performance point of view.
The only gains are lifespan of the cat, if you aren't using it (bypass fitted) then it isn't being worn out. And changes in tone and volume of the exhaust.
On a track there will be no tangible gains.