my sense is that when running press it is best to keep it at "fairly fresh" --- I usually reserve "getting tired" for players with very high stamina (90s perhaps) and other types of defense. Obviously there are other factors to consider - such as how many guys are in your rotation and if you are running fastbreak offense. It seems to me that my press teams are best when I can use all 12 guys. In that situation I run "fairly fresh" almost always. My current press team runs motion, so I may consider running slow-down and perhaps use the "getting tired" option on fatigue settings in some situations where I think my team is at a disadvantage.
I don't know how much the performance drop is, but I think it's fairly significant for the press defense by the end of the game. Of course the other factor to consider is how good the bench players are. Generally I think press is the best defense for coaches that are able to get good depth on their squads. If there isn't much drop off between the starters and bench guys, press seems to really help.