No it shouldn't be a problem. Heavy coolers are undoubtedly going to put stress on the graphics card PCB and the PCIe slot itself, but there are much heavier coolers than that and I've never heard of it damaging the card.
If you prefer to have your card sitting straight (either for aesthetics or just to be safe) then you can buy backplates for the card, which increase the rigidity of the PCB.
http://www.coldzero.eu/r9-200-series/1818-coldzero-backplate-xfx-r9-280x-double-dissipation-1.html
Woody