UPDATE: See test results below and why you underclock memory, overclock core clock and undervolt VDCC.
how much are you getting from a single R9 280x?
I have three different models: VaporX, TriX and Asus OC edition - most of them are clocked at 1070 core / 1550 memory. The maximum hash rate per card is 23 MH/s for the VaporX, TriX. The Asus cards are doing 25 MH/s max. I tested that on Win8, Win10 and Ubuntu.