.....it really depends on the cat.

there are some good high-flow cats - like the renault cat used by hytech, which have virtually no power losses compared to a test pipe.

however, the majority of the cats on the market will sacrifice ~5whp or so - even more if you're making over 200whp.

that being said, the tone isn't too noticably different - maybe a few decibels.

and you can't pass emissions sniffer with a test pipe.