I can't see why a reducer would lower the trucks hitch rating The same forces are being applied to the hitch whether the reducer is there or not. In other words the hitch doesn't know... WHat will reduce is the rating for the 2" shank assembly compared to a 2.5" shank.
My 2012 GMC also has a 2.5" hitch/reducer and there is nothing in the literature I have found saying hitch capabilities are reduced when using 2" shank/reducer.
If however, I was approaching those kinds of weights then I would probably spring for 2.5" shank/hitch assembly