EACH fuel company offers a 98 RON fuel above regular unleaded and each makes claims about the benefits of their particular fuel. They aren't the same so trying a different brand may work, but I would use regular unleaded in your Magna. I believe the benefits of using the higher-grade fuel are negligible, which your results suggest.

Checked with Mitsubishi experts Des & Grays Automotive in Ferntree Gully and they suggest you check the engine mounts. The Magna has a tendency to chop out the engine mounts, the front one in particular, and the engine is then able to move further until it's hard up against the mount. In such a case the exhaust can also be hard up against its mounts, and the result is a harshness transmitted through the body. That's the most likely cause, but it's also worth checking the transmission. You don't say if it's got an auto trans, but if it has it could be slipping in reverse.