OK, here's my first test, as it were. This doesn't compare low- to high-iso, but it's some bracketed shots I had floating around.
7D, Samyang 35/1.4, canon-brand ND8, iso100, f/6 or so.
3 bracketed shots, -4/3 EV was 1/400s, 0EV was 1/160s, +4/3EV was 1/60s.
All processed the same in DPP, WB to daylight, standard style, sharpness 3, NR was 0/0, ALO PIC etc off.
100% crops from the centre of the frame.
The 1/400s shot was bumped up +1.33EV, the 1/60s shots was cut down -1.33EV in DPP. Certainly the 1/60s cut down looks better. As far as noise goes, it's best to just have everything set as high as possible to on the point of 1 pixel hitting its saturation point, no blowouts and least noise (but it's not always practical).
It's raining and grey here, so I can't go shoot a nice-looking bracket with variable ISO to answer the OP, maybe i can find one floating around in my collection later.