Used to have the same problem and (knock on wood) when I switched to 5w30 (basic chevron dino lube I buy at Costco) I don't get nearly as much smoke in the same traffic as I used to with 10w30 or 10w40.

Not sure why but as the saying goes... "Why ask why"... I'm just happy it's not so smoky anymore.

As for if it's worth replacing them, depends on a number of factors IMHO. Here in Cali, I'll probably need to replace my cat in 2 years when the inspection is due again so I won't bother replacing them until then but if you don't be prepared for the cat to be more easily clogged by all the particulate matter going through from the burning oil. Since yours is a 97 (mines and 96SR) you probably aren't far behind me as far as your cat is concerned... but then it also depends on your state's testing requirements.

HTH.