Most companies that make a separate, standalone wedge shaft such as the Spinner will design such shafts so they have the same overall stiffness as a shaft that would be installed in an 8 iron in the set. So they are in essence made to be 2 clubs softer in their overall flex. Thus if you have the DGS3 taper tip, you would use the 8 iron shaft in the wedge. If you had a parallel tip version, you would use the 8 iron tip trim amount for the shaft being put in the wedge.
What you're talking about in terms of lower flight with more spin is a MYTH - you can't make a shaft that offers a lower launch angle AND higher spin. To make a shaft offer lower flight means you have to make the shaft stiffer overall or make the tip section stiffer. When you do that, you act to LOWER the spin on the shaft, because stiffer shafts always spin the ball less than more flexible shafts. Can't be any other way.