2012 Jimny exhaust problem.

I have focused on the possibility of a blocked exhaust simply due to the hissing noise when you turn the engine off. Its possible nothing is wrong with the exhaust, but the Jimny engine is "happy to rev" and although not quick they can get up to speed ok, but are not comparable to a diesel.The good news is the CAT / Exhaust is relatively cheap, and you need to look at the leak you have anyway. I would drop the exhaust and check everything, debris in the CAT and / or collapsed rear silencer box.If you know someone local with a Jimny get a drive or get them to drive yours, simple to compare any performance issues. 
Good luck
Mike

Edit - just to add short shifting and short journeys can be the death of CATs and exhausts, drive it like you stole it every now and again (within the law).

Should I take a short video that might aid the theory? The exhaust system is pretty rusty looking too. That being said, what is the rpm range when I should shift to be more efficient? Around 3000?

Another thing that drew my attention, the small CATs with oxygen sensors in them, are they actually CATs or something else. Or am I confusing the exhaust muffler at the back for the CATs in front and middle?

There is one cat in the manifold and one in the next box with the sensor.
First cat sometimes disintegrates and clog up the next.
However the hissig noise can be an obstruction in the cranck case ventilation pipe from the side of the engine to the manifold. There is also a valve there could be blocked.

Alright, looks like I'll be removing the manifold and try to get to that breather system as well. Get new gaskets too because somewhere is a leak. What problems would be associated with a blocked crankcase breather? Thank you for informing me.

Before you start taking it apart any chance of getting a volunteer to get under the vehicle when you switch it off, try to narrow down the source of the hiss?
​​​​​​May be worth paying for 15 minutes on a car lift if you don't fancy crawling around.

I can try that. I replaced the leaking rear diff oil seal myself so I don't mind crawling around underneath and getting my hands dirty, love it. I have jacks and stands to get the job done.
The hiss clearly came from somewhere round the exhaust manifold. I could easily hear it and if the engine didn't run long, there is no hissing at all. Almost as if pressure builds up inside exhaust the longer the engine runs which is obviously the case right? Thanks for guiding me on the right path.