They shouldn't be restrictive if the vehicle is fitted with some of an adequate size.
I started getting error codes for the cats. After pricing the cost of the Y pipe in Oz, I just paid a small premium & went for a full Ripp Long Tube Headers exhaust. I guess I could probably have had an exhaust shop fab something up with after market cats, but I haven't had much success with the workmanship of any of the places around where I live.
The Butt Dyno says, "meh". I can't tell any difference in normal driving, (except for the sound). Maybe a bit more top end, where I rarely go...
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