The “super elite” marathoners would not do well at Comrades, and the common assumption that a marathoner could just move up and do well is a fallacy that ignores the physiological differences in the two races. Though I don’t disagree that it would be entertaining, because it would be nice to see Kipchoge finally fail at something! As someone mentioned above, Comrades, unlike any other ultra, has had a glut of very accomplished marathoners who didn’t pan out. A lot of good South African marathoners (sub 2:10) in the 80s tried and couldn’t hang with the top Comrades guys over the distance.
The best way to think about the relationship between marathon ability and fast ultra ability like Comrades (technical trails are a different story) is like the 800 and the 1500. At the highest elite end, the physiology of the guys doing well in each precludes them from doing well in the other. Likewise, for Comrades and the marathon, a great Comrades runner has to have good (but not world class marathon ability) much like a Olympic medalist in the 1500 has to have good (but not Olympic finalist potential) in the 800. Kipchoge at Comrades would be Rudisha in the 1500. Entertaining? Yes. Successful (without a lot of specific preparation)? Probably not.
The likes of Kipchoe and Bekele would, as someone mentioned above, either DNF or fail to crack the top 10 without a long block of very specific preparation, and even that may not guarantee success. The only elite marathoner right now that I’d fear at Comrades is Yuki, but even he cracked pretty badly on a much shorter ultra a few weeks ago.