With your car, you have a turbo, so less backpressure is good. Just removing and replacing your cat with a straight pipe is not going to cause a dramatic loss in back pressure, the exhaust system and stock header will provide plenty. With a naturally aspirated car this is more of a concern, but you have a turbo so freeing up the exhaust as most as possible is the way to go. You could see a 2PSI gain on your turbo by replacing the exhaust with larger piping from the downpipe back. If you replace the downpipe as well and do a full cat-less turbo-back exhaust you can see 20hp easily.
1990 Ford Probe LX 3.0L V6 :s6:
SEE IT HERE:
1989 Mazda RX-7 Turbo II 13B-T "FC3S"
Turbo RWD Rotary Powa!