Well what I find likely is that it's no coincidence that Tao Pai Pai, Gohan, Tsuru and 22nd Budokai Roshi are all in a somewhat similar plane of power.
Tao Pai Pai is implied to be much stronger than he was when Goku beat him, Gohan is an old ass man and how fucking old is Tsuru?
They are all old bastards that are stupidly strong. If anything, it suggests Roshi's lack of training more than their weakness imo.
It doesn't even seem like Tsuru has fallen off all that much even after all these years. He must train pretty diligently for centuries to even maintain a power anywhere close to his power in his prime.
All in all I'd probably have something like this...
Mutaito > Roshi/Tsuru/Gohan ( All in their Primes ) > Tao Pai Pai ( Prime )
I think in their prime conditions, they can all be around Goku's 22nd Budokai power or better.
Hell, Mutaito in his prime could even be stronger! As an old man he already was a good deal stronger than probably Goku at the 22nd Budokai.