well, Im not exactly an expert on this, but I have had alot of experience working on turbo cars, and from past experience, it really depends on how much boost you are running. The more boost, the larger diameter pipe you need to run, thus, allowing better flow to the turbo. Since boost is basically forcing air in and out of your engine. The biggest restriction on XF's is probably the cats, since there are like 3 sets of them. But since you can't remove those due to emissions and OBD2, your only real choice to open up your exhaust flow would be the muffler, and I can tell you right now by looking at the factory one, its a piece of junk, its heavy, the exhaust flows into, out of, and then back into, and finally out of the muffler. I installed a custom exhaust on my XF, I had my best friends dads drag race team install a 8 Inch (in length) 3 Inch Inlet Single chamber to a dual 2.5 inch outlet drag muffler, and I could tell a huge difference. Especially in throttle response. I even ran it with straight pipes for a while, without a muffler, and it really opened it up, but the neighbors didnt like it to much. But, everyone has their own opinion about sound, performance, etc.

With a muffler like the stock one on the xf, you create a bottle neck effect, causing back pressure, which in some cases is ok, (small displacement engines), but the xf 3.2 is a fairly decent displacement engine.

And the less bends you have the better. Take your car to a shop that has a mandrel pipe machine, rare usually, but makes for a much cleaner free flowing exhaust.

These are just my 2 cents. I wish there was a decent set of headers for the xf that didnt cost so dang much.

Does anyone know how interchangeable the Mercedes parts are with the xfs as far as engine goes. manifold etc?