yeah, just have a guy upstairs review everything except balls/strikes (however, HBP or foul tips can be reviewed). no need for managers to have limited challenges. speeds game up by removing right of manager to go out and argue calls too. even if an ump makes call favorable to the D, a pitcher could try to throw the next pitch quickly but batter has right to step out of box and give review ump more time to get call right, so unlike football where QB hurries to the line and gets the snap before replay can rule, the team that desires more time for ump to review a call can easily provide a slight delay in the game. i just don't think replay will cause any noticeable slowing down of the game, and actually can speed it up by taking away players and managers right to argue calls and delay games by 5+ minutes. if they do, immediately toss them and a big fine will stop such behavior in a hurry.