Do you think the high ISO ability of the A7s sort of negates the benefit of the IBIS on the A7II since you can just increase the shutter speed and then increase the ISO to compensate?
Yes, definitely. 
Funny thing, but one reason I would love IBIS in the a7S II is to help the EVF out when using longer lenses. My Zeiss 135mm is a bit of a pain to view through the EVF since it's unstabilized. I can't imagine trying to use Charlie's unstabilized 200mm f/2! Another reason IBIS would be great on the a7S II is it would make shooting long lenses in the dark even easier; just would give a bit more creative control over shutter speed and ISO. Nonetheless, yes, I think the a7S' high-ISO prowess can overcome lack of IBIS.
But, yes, the a7S high ISO prowess is insane. This is a turd of a photo, but was just an experiment to shoot a dark street scene (at dusk) handheld at f/8. It is at ISO 8000 and after I ran through DXO Optics Pro 10's "Prime" noise reduction (which is the most amazing raw-conversion noise reduction available) it is noise-free with excellent detail.
FE 35mm f/1.4 ZA (this is an old photo from before I returned the lens):
Sony a7S



Give me a year and I'll be the guy whose gear rarely changes, but right now: constant flux/constant second-guessing.
