Personal investments aside. The way I would determine fairness is to make sure everyone has the same advantage over the players at the lower end of their range.
Power of Player B - Power of Player A = Z
P(C) - P(B) = Z
P(D) - P(C) = Z
....
The difference of power is the same. The way you work this out in game terms is a balance issue, but I agree with the concept.
For balancing, my opinion is...
A) I feel P(220) - P(207*) ~ P(207) - P(165).
B) I don't know if P(207) - P(165) ~ P(165) - P(lower range of 165)
I'll argue that A is true, but I can't speak for B. If statement B is not true, ranges should be adjusted by either..
1) P(165) - P(lower range of 165) should be increased by lowering the range even further.
2) P(220) - P(lower range of 220), P(207) - P(lower range of 207) should be adjusted by tightening the ranges to represent the same power difference as P(165) - P(lower range)
Levels are not created equal. Therefore, looking only at the level difference is not entirely representative of the difference in power.
The range with the biggest problems imo is 171-206.
*NB 207 is not the lower range of 220, but it may as well be. Anyone lower has no chance in hell against a 220.