exhaust hole = this problem?

[Antigenus]

I have horrible hole(s) in my exhaust. The 2 to 1 cat that the passenger side exhaust pipe goes into has a slash thru about 80% of the pipe and then behind it, it has a ton of holes like a shotgun hit it....i tried to patch it temporarily but its not working....could this be why I am getting horrible gas mileage? its losing a ton of exhaust around that area...