[1speedbike]

I don't understand. Isn't a "full exhaust" something that includes downpipes? So shouldn't that be the "best"?

Also, what's the difference between "dp" and "keep stock but take off all the cats". I am assuming that your car is N55 so there are no cats in the midpipes. So that would mean that both these choices involve changing the downpipes in order to get rid of the cats, so they are equal.

Downpipes will free up the most power. A set of catless downpipes on a stock car will probably free up to 15-25 horses, and more if you have a tune. It's a broad range because there are both 2.5" downpipes (stock size, but with no cats) and bigger sizes such as 2.75" (aFe) 3" (AR, AA, others). The 3" pipes will obviously be more free flowing and allow you more power, but will be more expensive. Also, consider getting a thermal coating on the downpipes to cut down on the heat.

An axleback + midpipes will free up maybe 15-20 horses at the most, even tuned, but will save more weight in the rear. The stock tubes are 2.5 inches. You can "upgrade" the midpipes and axleback to a bigger size, but the stock connections are 2.5 inches so it will bottleneck anyway and have no effect. You can get bigger downpipes because that is the fisrt thing the air from the turbos hits and letting this flow freer before a bottleneck down to 2.5 inches is still beneficial.

If you are running catless downpipes, you should run either map 2 (the more aggressive map) or map 5 (the autotuning map).