140mm rear rotor for bike packing Currently my bike has cable actuated disc brakes, which I will upgrade to Ultegra 8070 callipers and RT-800 Ice Tech rotors.  I use the bike for road, some gravel and bike packing.
The current rotors are 160mm front and rear.  The rear rotor is a pain when I remove my rear wheel: due to the rear derailleur I need to move the wheel slightly forward to remove it and I end up scratching my frame with the rotor (the non-drive chain stay, even when I am careful). After 2 years of doing this, the frame has lost quiet a bit of paint...
Because of this I am thinking to put a 140mm rotor in the rear, rather than replace it with another 160mm rotor.  But I am nervous about this for bike packing use.  I weigh about 80KG and worst case my gear would be about 10KG if I would ride with a pannier.
Obviously I don't want to get in a tricky situation by overheating my brakes, but I am also fed up with the scratching and want to apply some clear coat on the exposed aluminium.
Q: Should I be fine with 140mm rotors on a (medium mountainous) bike packing trip given my weight?
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.