I actually use the PS88, having used the Aero Pro, BLX90, K90, N90, and PS85. You are correct when comparing the Aero Pro to the BLX90 in terms of power. However, if Fed sought more power in a larger frame, there are a wealth of options on the market. My argument wasn't speaking in terms of power when referencing the Aero Pro, but of precision. With the Aero Pro, I didnt have to be precise in terms of footwork to hit a great ball. I could show up a little late time and time again and just let the racquet whip and crank a winner. With the BLX90 or any players frame you have to have perfect footwork almost every time to hit a great ball. Very few of the top 10 even use player frames.
What you are describing in terms of setting up late is playing with a racquet that is heavier and swings slower, not so much headsize IMO, this would seem to be subjective to each individual player and their strokes and conditioning. What helps one player may not help another.
Federer generates his topspin on the forehand differnently than most pros do, he swings through the ball and turns his wrist over at contact, so he doesn't need a big spin window or whippy frame to hit his forehand. That said, I don't see how it would help more than likely he would lose some control and power on his best shot. Think of his forehand as a flat stroke in terms of production,eventhough the end result is a tremendous amount of spin.
I too have played the various Wilson 90's and the Babolats. Strokes with a flat swing plane were much harder to control with the babolats. the ball gets lost more on the bigger head.
They only offer an advantage on steeper topspin strokes with their bigger spin windows.
Federer doesn't swing with a steep swing plane to generate spin, he is unique, so I don't think a bigger racquet would help his game. Better to change strategy and play more aggressive if he has lost a step than to play the same and hope a bigger lighter racquet can cover for poor footwork, IMO.