Here´s my take on stamina and a little bit of Game Shape.
My experience is that Stamina is a little more important then most managers choose to believe.
I believe (its not my religion) that Stamina helps your player to perform better over the entire game(more in the end ofcourse), not just in shooting and defending but also in avoiding fouls to some extent.
I have a strong feeling that my high Stamina (above Strong in average) is a big contributing part of my success of late.
My guards are often referred to as "overachievers", since they have "low" salaries. They perform close to their best every night, much due to their high stamina i believe.
Last year league-MVP Nikos Fanoureas (only 91k salary PF listed as C) has his Stamina (and Driving) to thank for that.
BUT you cannot stick your head in the sand when it comes to Game Shape.
It´s even more important when it comes to impact on the players performance.
Managing minutes and Game Shape during the season (or at least the playoffs) is crucial to my team and a MUST if you are aiming for the silverware in a tight competition.
In closing, if you think your players and team are underachieving, try getting your stamina up a couple of steps, then re-evaluate. Still underachieving? - you got the wrong players for that tactics!
My bet is that you should se a increase in your players performance, leading to a better result for the TEAM.