Nice article!!

But this is not what you're testing. You're not determining the effect of imbalance alone, but imbalance + particular position of pieces. To determine the effect of imbalance you should study how does not having a certain piece since the beginning affect the result of the game.

Also, being a pawn up in the opening is much more different than being a pawn up in the end game. It would be interesting to redo this analysis but splitting by opening, middle game, and end game.