Guys my stock gigabyte windforce 280x only draw 260H/s currently, is there anything i can do to increase it?
I think it is quite resaonable speed.
R9 280X SHOULD HASH ABOVE 300Sols/s--
My 3-year-old PowerColor 280X cards hash at 320Sols/s. They are overclocked at 1100 core / 1500 mem (1030 core / 1500 mem stock). The overclock setting is stable for months at a time. Any higher clock settings are unstable. These cards are faster than RX 470 cards, but use more power.
Is your power supply enough? My cards run on FGLRX drivers. If you have updated your drivers to AMDGPU Pro (v17.4.3+ is supposedly compatible), your 280X cards may not perform as well. If your cards are running hot (80 deg C or more), they may be throttling.
I don't have too many tips, but those 3 items (power, drivers, heat-throttling) are worth a look. You might also try changing the riser, or mounting the card directly on the motherboard. Good luck! --scryptr