High level players play axis a LOT more than they play allies (~roughly 4 times as many). This means that only a fraction (at most 25%) of top-tier axis games are against top-tier allied teams. The rest (75% or more) have to be played against less competent teams; because after all, for every game with axis, there has to be a game with allies. Ergo the axis have a very high win rate in 4v4. Conversely, an high-level Allied team is very likely to face a high-level axis team because there are 4 times more people of their level playing with axis; ergo the high level allied players only have ~50% winrate.
This is logical but you can see the amount of games played as axis / allies by top 200 players in the charts and whilst less its about 2350 axis compared to 1360 allies (4v4 - the worst imbalance of win rate) so the imbalance isn't as large as you say. Also consider another chart (I cant recall where it was) showing the win-rate curve across the entire playerbase which showed a steady but shallow rise until the top 50 where it rose exponentially - the allied teams being played by top 200 will have a broader range of skill than axis opponents but not necessarily by that much. I don't agree with the OP suggestions - but I do think as games get bigger axis gain a large advantage. I think to pull big games back towards a balanced center line needs a different approach - looking at resources and synergy between OH and OKW