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I've had the single cat on for about 2 years, with no problems. It's mated to a 2 1/2" Y and long-tube headers. Cost about $200. Supposed to flow really well; I know the car got a lot louder after I installed it. Of course the old one may have been clogged somewhat.
703000 random super high flow stainless steel cats w 95 390 ZR1
505 rwhp with cats & B&B. the cats flow exceptionally well, pass emissions and don't really restrict power. I drove the car without the cats and didn't notice any difference in top end.
I suspect they hurt HP less than 5 rwhp. unfortunately I didn't test before they were installed.