chris, you open the intake and the exhaust up to gain horsepower.  this is
why all the performance exhaust systems you see have larger diameter piping, and
more free flow (louder) mufflers..  and the intake systems are opened up with
a cone filter or free flow air filter element rather than a stock restrictive
airbox... running without a cat will not affect your engine in a negative
way.  it will actually add a few ponies if you want to really get technical.

chris