Not sure what you're implying Road Rage (from your last) but one member down
here got 7 rwhp from his test pipe. I was there to see the dyno runs during his
E Manage install. The test pipe was the last thing to go on following a full
tune and just from its install with no additional tuning, he definitely saw a gain.
Free-flowing cat's are an oxymoron. They are "free flowing" as far as cats go
but are certainly not as performance oriented as a test pipe with the appropriate
sensor installed.
However, the sound from the test pipe totally changed his S2000 and it sounded
horrible. Gone was that "S2K" tone and the car at WOT sounded more like an ill
modified Civic.
He took it off.

 

I was clearly stating my opinion that bypassing the cat to get a handful of peak HP is a bad tradeoff against the pollutants that are produced. The term "test pipe" is a silly one. It was used in the early days before people had pressure meters that could thread into the O2 sensor bung to determine if the cat was clogged.