If you ever have the (increasingly rare) experience of following a gasoline car without a catalytic converter in traffic for any distance, you'll be reminded instantly of what good thing these are. There's absolutely no way breathing raw exhaust can be good for you, and it's really kind of antisocial to impose it on others, especially for minimal horsepower gains. Here in Atlanta for instance we've got an increasing problem with photochemical smog and ozone, proven beyond a doubt harmful to children and oldsters. Despite what the naysayers may wishfully claim, photochemically reactive hydrocarbons and nitrogen compounds such as are found in untreated automobile exhaust would be a even larger exacerbator of this problem were it not for catalytic converters and other smog control tech.

Here's a useful site that describes photochemical smog:

http://www1.ldc.lu.se/iiiee/IMPACTS/PHOTOSMOG/PHOTOSMOG_HOME.HTML

As a Volvo owner you should be happy with this site, since it's Swedish.

Just my two cents. Flame away!