Exhaust leak?

Hi, My IO has been running rich for the last few weeks. Im only getting 300kms per tank, wheras in the past i was getting closer to 500. Alongside this its running a bit rough and shaking in lower revs when accelerating, and idling. It also runs slightly worse when the aircon is on, not sure if this could be related to am exhaust leak? I have attached a photo of a crack on the centre muffler, which i think is likely the culpurit. There is a lot of surface rust on the exhaust but only one obvious crack/hole. 

Any advice on if this is likely the only issue for such a small crack? and any advice on what the best fix is?