I am buying a used set of irons. I have used the NS Pro 950 GH shafts on Mizuno irons . The shafts were Stiff. My clubhead speed in under 90 mph on irons.
The question is -- Should I be using the R Flex with clubhead speed under 90 mph?
I use the 950GH stiff on my irons. My 6 iron swing speed is 83-85, so you are fine. They are nice shafts but are softer due to the light weight. I was fitted by a GD Top 100 fitter, BTW.