It is logical to think that wrapping an over grip around racket's shaft increase the stiffness since more material has been added to the shaft. The question raise here is; however, does it really increase the stiffness? will you feel the difference? The answer are Yes and No. Why?
Speaking technically, there are mainly 2 factors contribute to the stiffness of the shafts one of them is "Material Modulus of Elasticity(E)". Grip> rubber>E,rubber 0.01 GPa where as Graphite shaft ,Egraphite 1000 GPa. If we need to increase the shaft's stiffness 1% , roughly estimation , then we would have to wrap around more than 100 layers. The point is the effects is extremely small and it is impossible to notice the difference.