I have two thoughts:

1) if an O2 senor was bad, your check engine light would go on (assuming that the light works...check its operation during the "self test" when you start your car)


2) Some back-pressure in the exhaust system is GOOD. It actually helps the engine perform better by something called the "scavenge effect"....ask Beav to explain it to you. Anyway it also helps with gas mileage. Yes, in some cars that have too much back pressure releasing some of it could help the cars performance...but by cutting out the cats you probably lowered the already low pressure exhaust system of the SVX to the point where there isn't enough pressure to operate properly. Case in point: my younger brother recently took the front pipe off of his 90 Legacy to see if it would go faster without any exhaust. Well it did make the car A LOT louder (it actually sounded cool) but it was SLOW AS HELL. Why? Without a certain amount of back-pressure the engine couldn't perform correctly. It also ate through gas like it was going out of style.

Hope this helps

Dave