Exhaust question on my 90 MarkVII

If you get inspected yes, if not no.
It really don't use any horse power but not needed if you get rid of the cats, or replace them with newer style cats.
Seriously, if the car is stock your not going to gain much by getting rid of the cats, good headers and a cat back will do, I would look at a rear gear change too. Now that will pep it up more than changing out the exhaust. JMO