Volvo: o2 sensors and cat converter, o2 sensors, good reply

Question
Hi Ray.
You gave a really good reply to my last question about a compression test re: 1994 850 volvo. Though it has good compression and runs smoothly, the engine seems to be quite noisy at times and lacks power. It never stalls.  

I was hoping you'd be able to tell me
whether replacing the 02 sensor or cat converter on the car would improve performance?  
Do you need to do both jobs at once?  
Is there any need to replace it?
What are the tell tale signs of trouble with the 02 sensor or cat converter?
Sorry i know i've asked a lot of question.  Thanks very much for your help!
Leigh


Answer
Leigh,
If the O2 sensors are bad, it will set fault codes in the powertrain management system. If there are fault codes for the O2's then there is a very slight possibilty that there is a power loss due to the O2's, more likely their would be fuel mileage issues.
If the cat is clogged that will cause power loss. However that should also set some fault codes like fuel trim codes.