[84LX89GT]

You won't have any problems with an '89 Mustang if you remove the catalytic converter. I don't have any of the 4 on my car (different engine, same management system). On newer cars they have catalyst monitors that check the performance of the catalytic converters, but that is on the '96 and up cars (at least for the GT's).

As for compression, that's just how much your piston squeezes your air fuel mixture in your engine, i don't know why someone would tell you it would change that at all because it's unrelated.

It will effect the emissions compliance of your car though, and carbon monoxide and hydrocarbons will increase, but it will probably run alot better and have more power. If you have to get it emissions tested a bad/plugged converter will make it fail, just like having no converter, so at least this way you'll be getting better gas mileage and have more power with no more noise.