according to nicehashs calc you go from 285 to 280 sols.. I assume that is default settings. So Overclocked/modded I guess they perform about the same. Why u want to change? To lower consumption?
280X CARDS PERFORM BETTER--
R9 280X cards get 315-320Sols/s, and RX 470 cards get 295-300Sols/s. R9 280X cards run hotter and use more energy. None of my cards, R9 280X or RX 470, are BIOS modded. --scryptr