Cat is also not "overpowered by nature". Cat was outright weak in r48, and the rankings reflect that.
Furthermore, your examples are bad in no less than 3 ways. First, the outcome of the calcs you propose is plain wrong. Here's De vs
400k Viper (40% and 14%) and
50k Roach (35% and 13%). Second, Ghost flak Pegasus, so the Viper is pretty bad against De teamups. Only the ETA helps them. And finally, for some reason you seem to believe that your De teamup should be equally strong against all races and all fleet compositions.