Compare your calculated stresses with the yield stress for the hardened material. If the stresses are below the yield stress, then a linear analysis is valid and increasing the hardness will make no difference.
If the stresses are above the yield stress, then either a) your parts will yield, and you could do an analysis with non-linear material to see the effects; or b) you should increase the hardness (or decrease the stresses) to make the yield higher than the stresses.
People often "feel" that hardness should influence the elastic behaviour (stiffness) of a steel, but it doesn't, certainly not at a macro level.
http://imechanica.org/node/2285
"There are no apparent changes in elastic modulus in metals that have undergone different hardening treatments"
Hardness varies with Young's Modulus, but YM does not change with hardness for a given material - "Elastic modulus is an intrinsic material property and fundamentally related to atomic bonding. Hardness is an engineering property and for some materials it can be related to yield strength."