I think you should win or lose points according to both players ratings at the START of the game, not the end...
ie if you take on a 1600 plus rater...but at
checkmate he/she has dropped a few hundred due to losing other games, you don't get the relevant credit for taking up the challenge!!... i know it can work the other way..but never seems to with me !
This system also encourages people to be tactical with the timebanks...ie delaying wins or loses at timepoints most beneficial to their rating.
Hope this makes sense