Iits also to do with how soon he will get subbed out.
eg: My PF vs my C
PF has Strong Stamina
C has pitiful
When starting, my PF tends to get 40-45 minutes per game - that means i get a lot of good production from him.
My C, tends to only play around 30-35 minutes when he starts.
The way substitutions work, is that - if the coach thinks that a benhc player can do an immediate better job than a starter, because that starts stamina is low - then he puts the bench player in.
So going back to your question - how much better will he play? He will play more like at the start of games, for longer, if his stamina increases.
Make sense?