So much of this shaft flex stuff is about pride and being macho. At 84 MPH you might even do better with a senior shaft but no one would want to admit that because they think their manhood will be in question. Hit what ever shaft gives you the best best combination of distance and control. At 84 MPH there is no way you can be getting the most out of a stiff shaft.
At 93-94 MPH I am between a Stiff and Regular and have 2 Drivers one with Regular and one with Stiff. I find that with the Regular I don't have to try and swing as fast as I can and hit the ball with a lot more velocity and hence more distance by leaps and bounds( as much as 30 yards father). The control is similar.
Don't let your Ego get in the way just because if your playing partners are all playing stiff. If you want to get your drives out near them go with a lighter shaft.
If my swing gets faster as I play more I will go back to the stiff.