I think you can turn some poor performers around. It is hardest with Scouts/slow trainers though. I got a player off a friend, that last season didnt do so well for him. I wanted him cos he was a lefty, and he looked like an okay trainer for his age. It took a week of adding skill points to see him improve, and he has gotten better as I add points. The hard part is figuring how you need the points, for me, I added a bunch to physical. If his attributes are already similar to a player who performs well, I try adding to his primary skills (off or def) for awhile. But like I said, with a slow trainer that's kind of tedious. If they're really poor performing after a couple weeks, time to sell them for me. Because, there are players that just won't work for my team sometimes.