I have had both the AR Design Catted and non-Catted downpipes on my car along with all my other mods at the time.
I did ZERO dyno runs with this car, so I have no hard evidence but only my opinion about the two.
From Stock to the aftermarket pipes makes a helluva difference. Second most important mod for your turbo Bimmer.
With the catted pipes, it felt as if I had a lot more torque when launching my car from a stop, and even from 40-100mph.
It was a lot quieter than the catless pipes during normal driving conditions.
It didn't have the exhaust odor of non-catted pipes.
However, even with 'racing cats,' I still ended up going to OEM downpipes in order to pass emissions. They will not pass in the state of California, unfortunately. I only passed after switching back to stock from failing the first time.
Catless "feels" like it has slightly less torque down low, but has a lot more pull up top.
You can hear the turbos scream when going WOT without resonated midpipes. (It almost sounds like a freight train)
My car shoots flames out the exhaust when shifting during WOT.
(Dunno if this is common)
The pipes have a deep rumble even if running OEM cat-back exhaust.
It leaves an exhaust scent in the air but not overwhelming.
There is more but I cannot think of anything else to type.
It is 2:25am here and I am heading off to bed now.
P.S.
Anyone want to purchase AR Design Catless Downpipes (N54)?
I'm going catted with my 1M.
