No catalytic converter?

Hey, since my exhaust system is kinda rusted on my 1999 mercury cougar i was going to get my friend to build me a new one, i believe i can run two pipes straight from the exhaust manifold and i was wondering if my car will run fine if i just don't put the catalytic converter back on. Will it get worse mileage or something, oh and i could care less about the legality of leaving it off, thanks