The hydration rate of the deuterium-substituted styrene should vary very little, if at all, from that of styrene itself, because the deuteriums are not transferred in the rate-limiting step.
A small effect of isotopic substitution occurs in this case because of the differential effect of deuterium and hydrogen on the rehybridization of carbon in the transition state. (The carbon bearing the deuteriums rehybridizes from sp2 to sp3 in the rate-limiting step.) However, this effect on rate amounts to only a few percent. Effects of this sort are called secondary deuterium isotope effects.