What is your water heater temperature set to? As fall is starting, I'm guessing your cold water temperature will be dropping some, and make it worse through the winter into next summer. As you extend that set screw out, it will hit the white plastic stop sooner, limiting the amount of hot water you can get. It appears that it is 'in' as far as it can go (allowing maximum hot). There is likely either a slot on the bottom side, or an hex requiring an Allen wrench to turn it.
Are all of the other fixtures able to get much hotter water? If not, you may have a problem with one of the valves leaking with a cross-over, diluting the hot water supply to the shower. The water will flow in the path of least resistance, so it may not be everywhere, but you may have some other valve with a cross-over. You may notice this by having the cold water warmed up, too, in other places.
I don't know if that model has a pressure balance spool valve, but if it does, it may be partially stuck, limiting the amount of hot water available. If that's the case, a new one should fix it.
