In high school my friend put the cat back on his 90LX because it was cheap and he was tired of his factory exhaust. It was off for a Flowmaster(2 chamber) exhaust in two weeks.The ultra flows actually made his car feel slower.

It really felt like it lost hp. No the mufflers weren't backwards.
The Flows really woke his car up. With flows, bumped timing, a K&N(silencer ripped out), and some very agressive driving he went 13.99 his first run. Then it was 14.0's-14.2's for the rest of the day.