"With my next set of rotors on the hardtail, I'm planning to run 180mm front and rear to see if I really miss the larger option in practice. The goal is of course not to save 100 grams, it's to stack a whole bunch of 100 gram savings together and see if my favourite mountain bike really ends up any less rideable."
At your published size and riding ability it will be horrible. It was the single worst decision I made (for spare part simplicity) at the 2016 Trans-Provence. 180 mm rotors have a place and that place is for < 75 kg riders.
As a 188 cm/ 93 kg rider I am 200-203 mm x 2.1-2.3 mm rotors front and rear for life.
If 100 grams is important then there are a lot of other areas one can shed 100 grams (a lighter saddle, non lock on grips, carbon 31.8 mm handle bar, good 2.4" tyres etc etc) that don't have a direct impact on ride safety and enjoyment.
My 33 lb 2020 Sight is still easier to pedal up a hill than my 26 lb 2003 Bullit was. As per your previous article weight is not a primary driver of efficient climbing.