A big part of being a major college head coach is recruiting--I'd say it's the most important part. The first two years Johnson was playing with Gailey's talent, which is not a complete measure of Johnson's ability as a head coach. The first two years showed that Johnson the OC is very talented. The last 5 1/3 are a better measure of Johnson the head coach. By the 3rd year, the coach "owns" the program.
Bobby Ross is an example. His last 3 years are a much better measure of Ross as a head coach than his first 2 years.
Actually, 5 1/3 years is not an "interval". It is a continuous period from 2010 to present. An interval would be comparing years 3-6 of Johnson (and leaving out 7 and 8) with years 3-6 of Gailey, since Gailey was fired after year 6.
If you want an interval, years 3-6 were worse. Johnson's FBS record for the 3-6 year interval: 23-25. For a comparison: Gailey: 27-21. O'Leary: 34-14. And Johnson makes almost 3 times as much as the other two made.
If anyone has an agenda, it's the people who say "Johnson has won more games in his first X years than anyone else". He has also played more games. But even more, he has padded his GT win record with 9 FCS wins (in 8 years). No other GT coach has come close to playing that many FCS teams.
