I see two kinds of VR "sickness".
1. There is sickness caused by locomotion implementations. Testing seems to indicate there are some people who will just never adjust to these. I really don't think FOV and eye tracking will fix it. I've experienced locomotion sickness in some games, and in all of them I've been able to completely adjust in 1 to 2 hours. That adjustment seems to be relatively permanent as well. I don't think it is a rare trait, but it seems that not everyone can do that.
2. There is this sort of initial "VR Fatigue" where the eye tracking issues might fit in. I've noticed a trend the first or second time people use VR for a real session (1 hour +), it is somewhat common (50%?) they get a little nauseous. I've really just been chalking that up to "new stuff is weird" and the heat the headset puts out. This sort of sickness seems to universally fade quite quickly for people, and could have to do with the head on a pivot thing. From what I've seen, most people find the sweet spot/head pivoting natural almost immediately.
TL;DR I spend a lot of time in VR and with people in VR. Eye tracking or larger sweet spots is near the bottom of the list of problems/discomforts.