140mm rear rotor for bike packing
I doubt it will make a big difference. A majority of the braking should be coming from the front anyway.  Most MTB's run a smaller rear rotor than front with no problems. 

There are a couple of things you can do will help if going to a smaller disc. If brakes are DOT fluid, bleed with a DOT 5.1 to give higher temperature tolerance (although being Shimano I expect the brake are mineral oil) and run metallic pads as these resist brake fade better than organics.  

The weight you are taking is not high - many riders of bikes would weight in excess of 100kg. Also when loaded for bike packing I would be surprised if you ride as aggressively as normal. 

Ultimately it comes down to you braking style. If you are heavy on you brakes, drag the rear brake and generally have little care for heat management in them, don't do it.  If you have good braking technique and are not riding aggressively you will not have a problem.