TO those who think HFCs or test pipes do not add to performance, you are misinformed. It does depend on the car and how restrictive the stock cats were, but most late model cars always experience a gain in exhaust flow, and these performance improvements are heavily documented.
Also, it is not just for FI cars. On the G37, HFCs are dyno proven as one of the single most gaining modifications with about 20whp improvements over stock. There are two of them, so that's one of the reasons.
Contrary to popular belief, most also see improvements in MPGs.
That being said, of course it will let more CO2 out of the car. However, the % of people doing this is so small, I doubt there is any significant scientific detriment on a large scale, especially not enough to offset the yearly decrease in nationwide emissions with the advent of more efficient and less polluting cars taking to the roads each day. I'm not saying it's OK, just that it's not this horrid thing a lot of people may make it out to be.
What you all should be worried about is CHINA. Talk about pollution.