The leaked specs all mention about a 20% increase in execution units. So it is basically 20% bigger GPU. It is also Gen 8 architecture which should again offer some extra performance. Intel a long time ago talked of about 40% of aimed for improvement. I guess clock rate should stay very similar, any gains from the new process will more likely go into power savings rather than the highest possible clocks.
The new architecture could be better than 20% more though. Given what room there is for improvement. Looking at what the smartphone GPUs can pull of on performance per Watt or Nvidia with Maxwell. Depends on how good Gen 8 turns out to be. No real news on that end.
I would say 40% faster is more like the minimum that can be expected.