The smell I'm talking about is unburned fuel. The owner-straight-piped AMG cars we get in the shop stink like an old carburetored car running rich. I don't want to get that stink in my daily driver.
As for the downstream O2 sensors, they've been disabled already.
I had the same overly rich fuel stench from my EFI'd 408 Magnum at idle. That was the only time I noticed it anyway. It was bad enough that anyone near the car would get all offended and try to fan them away while I cruised through.
I have a full TTI 3" exhaust, and welded in a pair of relatively short (7 " by 3" inlet & outlet with about a 4" thick body) metal matrix, high-flow cats just after the collectors. The metal matrix honeycombed structure is coated with the same rare metals as the ceramic matrix cats, but they flow 30 to 40% more.
It made a huge difference in cleaning up the smell. It's gone now, except when you first start the car. It takes a few minutes for the cats to get hot enough to "light off". It very slightly smoothed out the exhaust note, but there was absolutely no difference in power output. The little 408 spins out 630 ft lbs of torque with no power adder.
I may install them on my 500" RB if I can't get the idle to clean up more with some fine tuning. I think most of my problem is from the overlap I'm running on the cams, as the A/F ratios are nearly perfect.
