For reference, our very own W1zzard's figures state that: (Two figures given relate to Quiet / Performance BIOS)
PowerColor R9 290X PCS+ Idles at 37°C / 38°C and full load at 68°C / 76°C
AMD Reference R9 290X Idles at 49°Cand full load at 93°C / 94°C
His idle temps are easily within normal for a 290X allowing some leeway for variations in airflow, what version (reference, non-reference, etc) but that 290X is not working hard at all under whatever load he had it under.
With a fullcover block and a high-end water loop, my 290Xs load [F@H, 100% usage] at (Core, VRM1, VRM2): 51C/49C/43C and 50C/54C/43C. These same cards ran 95C under air, with 18C ambient (regardless of whether one or both was installed). A 50C loaded 290X on air is... unbelievable unless that happens to be out in the winter air.
Suffice to say, so long as the reason the card is as cool as his numbers show is that it's barely being loaded, then I'd say the GPU temps are fine, otherwise it's either not switching into 3D clocks or some other issue. If the obvious things that you and OP have been trying have no effect/prove to be fine, I would next look at the GPU drivers.