I'm kind of amazed that there really are "superb coaches", a scant few, who make a difference. I would have guessed most college HCs are "good", but not great, and the great ones maybe win one extra game a year they wouldn't have otherwise. The gap seems to be wider than that.
Particularly if you are talking regular season only, it may only be a one game difference. That doesn't sound like much, but realistically it is HUGE. Look at Michigan, for an example (ignoring 2020 because it was a goofy year):
In 2019 they went 9-3 with losses to Wisconsin, Penn State, and Ohio State. Heading into the tOSU game they were #10 at 9-2. With the loss they dropped to #17 at 9-3. Wisconsin and Penn State (both 10-2) were #10 and #12. Had they won any of the three games that they lost, that would have been a better win than anything that either UW or PSU had and put them easily in the top-10.
In 2018 they went 10-2 with losses to Notre Dame and Ohio State. Heading into the tOSU game they were #4 at 10-1. With the loss they dropped to #8 at 10-2. Notre Dame was #3 at 12-0 and Ohio State was #6 at 11-1. Had they won either of those games they'd have been a likely CFP participant:
- With a win over ND they'd have knocked ND out and finished 11-1 with their only loss being to tOSU.
- With a win over tOSU they'd have gone to the B1GCG at 11-1.
In 2017 they went 8-4 with losses to MSU, PSU, UW, and tOSU. Heading into the tOSU game they were just barely unranked at 8-3. With a win in any one of those games they'd have finished a respectable 9-3 with all three losses being to teams that finished ranked:
- tOSU was #5 heading into bowls
- Wisconsin was #6 heading into bowls
- Penn State was #9 heading into bowls
- Michigan State was #18 heading into bowls.
In 2016 they went 10-2 with losses to Iowa and Ohio State. Heading into the tOSU game they were #3 at 10-1. With the loss they dropped to #5 at 10-2. A win over either Iowa or tOSU would have put them in the B1GCG and gotten them a CFP berth with a win there.
In 2015 they went 9-3 with losses to Utah, MSU, and tOSU. Heading into the tOSU game they were #12 at 9-2. With the loss they dropped to #19 at 9-3. A win in any of those three games likely puts them in the top-10 heading into bowls.
Winning one extra game per year over the five years from 2015-2019 would likely have given Michigan a couple of B1GCG appearances and probably a CFP appearance as well. One game per year may not sound like much but it makes a big difference.