Not easy to state. Flex is more a variable based on how the shaft is loaded in transition. Weight and launch characteristics are more relevant in shaft selection as we mature.

Like a sweet white wine, we don’t get better with age. If you can’t hit a stiff flex, the R flex in the same shaft won’t work, either. If you were fitted into that shaft with the R flex then it should be fine. The fitter may have seen something that can be adjusted through the S flex.

This is a question you need a fitter to answer. If the iron model, shaft and shaft length are what you, then you should schedule a fitting with a qualified Titleist fitter. Once you complete the fitting process, the fitter will be able to tell you if the modus 3 tour 105 stiff shaft is correct for your swing speed.