http://cfbmatrix.com/1st-year-head-coach-matrix/
I recognize this isn't completely comprehensive, but is far better than the statistics you decided to make up on a whim. Any team winning 6 games or less actually improved their record the following year with a new coach. Teams with 7, 8, or 9 wins the previous season also only regressed by an average 1.04 games, which definitely does not represent some sort of "significantly lower record" with a new coach. The only real effect was on a team that had won 10 games the previous season. BYU is not a 10-win team nor has been for several years now.