I've actually read the research on ski width, namely Seifert's manuscript (don't know if it's actually published in a journal), and the article by Zorko et al that is published in Journal of Sports Science and Medicine in 2015, that are both discussed in the Seifert lecture.
The Zorko article uses 6 male skiers between 20-30 years, from the Slovenian Demo team and competitors, using 3 skis 176cm long, 21.4m radius, similar construction, with 66mm, 88mm and 110mm width. They ran a hard snow course on a 14.5 degree slope, with 12 gates 30m apart and offset horizontally by 11m between gates. They measured knee flexion (bend), internal rotation and abduction through the turn, along with ground reaction force. The results were: ground reaction force was essentially identical for all three ski widths. In order to achieve this, knee flexion, internal rotation and abduction varied between the three ski widths, with the skiers making active adjustments to achieve the same ground reaction force. However, these adjustments "could bring the knee joint unfavorably closer to the end of the range of motion in transversal and frontal planes as well as potentially increasing the risk of degenerative knee injuries." In the discussion, it was suggested that wide skis might produce increased tension on the medial collateral ligament - the ligament on the inner side of the knee joint, and more speculatively, more risk of ligament injury in case of abrupt sudden increase in force, and increased risk of knee cartilage degenerative damage over time.
The Seifert study used one skier, Olympic Gold Medalist Debbie Armstrong, on 66mm slalom skis and 95mm wide skis and looked at activity in several muscle groups from waist to ankle, as well as knee flexion. She skied a course on a 22 degree average pitch groomed run with gates 15m apart and 4m offset, as well as free skiing. In the course, her turns were faster and with a greater edge angle on the 66mm skis. The conclusion was "Skiing wide skis substantially changes skier movements, muscle activity, and ski actions compared to narrow skis."
Both studies show that expert skiers ski differently between 66mm, 88-95mm, and 110mm skis, but they don't actually define where the boundaries between narrow, mid and wide ski are. In the Zorko study, the curves for the mid size (88mm ski) for knee flexion and abduction mostly lie close to the fat ski curves and separate from narrow ski curves while the internal rotation curve is more or less midway between them, so one could argue that 88mm is more "fat" than "narrow," as far as the knee is concerned. Without data on widths between 66 and 88mm, the best we can do is draw a line midway between the two, which would be 77mm, close to Seifert's 80mm. And finally, all this is predicated on skiing on firm, groomed snow.
One other potential issue for wide skis, as mentioned in the Zorko article, is the possible increased risk of ligament injury in case of abrupt increase in force, which may be more likely in uneven terrain, e.g. chopped up heavy snow, moguls, etc.
Since the majority of skiers are found on groomed slopes, one might therefore conclude based on this research, that the the shops should be full of 70-75mm skis. OTOH, who wants to be considered average...