P.S This is in a dual GPU rig alongside a 380x which running perfectly fine.
Using Claymore 11.3 on Windows 10, 8GB, INTEL G4440
I don't have a R9280 but I've a couple of R9270x cards, one a 2GB and one a 4GB and I get 480 h/s on both cards.
I've set on Afterburner, Core Clock 1200 and Mem Clock 1450 on 4GB card and Core 1200 Mem 1500 on 2GB card.
Afterburner, out-of-the-box, will not let me lower the voltage on the R9270x (probably on the R9280 too) but with this tweak to Afterburner I've managed ot lower the voltage,
https://rog.asus.com/forum/archive/index.php/t-81916.html