I actually did some testing with this invariance with my 5dIV.

What i did i took first picture of northern lights with settings like 4s iso 6400 f/1.8 then changed the settings to 4s iso400 and f/1.8. ISO 400 because apparently 400->5DIV is iso invarient. Then what i did on LR i boosted exposure that +4 and yes, the noise was close the same as taken with 6400. The dark areas on the landscape area do get some stripes what you get when boosting shadows too much etc. but the basics were pretty much the same.

I am wierdo when it comes to iso values when shooting northern lights that i don't use the so called base isos much i tend to use 2000/4000/5000 mostly when takin pictures. But what i need to test out still is that if taking the images like iso 800/1600 like i would do them with 4000/5000 and then do some +2-3 exposure upping, what kind of quoality i get there instead taking out some exposure from my bit over exposured ones.