I have the piece of kit equipped which gives extra stamina chunks and reduced rate of stamina use. .. I have used this the whole game.

I am near end-game now. I have maxed out stamina so the equipment is only providing reduced stamina use rate. However I have also been using Zeus's Lightning to upgrade stamina use reduction rate. I must be near 18%% stamina use reduction rate from zeus's Lightning alone.

If the two forms of reducing stamina use do not stack, then the equipment I have for staimina usage is doing nothing now.