Seems to differ form review to review, this one released today wiht latest drivers has the 980's ahead of the 780ti/290X:
http://www.kitguru.net/components/graphic-cards/zardon/sapphire-r9-290x-vapor-x-8gb-cf-review/
It varies from game to game, on some games the 980s are very rapid and some they are poor @4K. I suspect the same goes for the different settings that reviewers use too.