You must be kidding ?!
In all seriousness, I don't know much about cars, but I learned when I was a kid that if you have a muffler that resembles a block of swiss cheeze, or in my case at te time, no muffler at all, you will not get as much power out of it due to a lack of back pressure.
If I understand this correctly, I think you would lose out on fuel efficiency for the same reason.
Perhaps if you have a torch you could cut a muffler & cat converter off one vehicle at the junk yard and then weld it onto yours ?