The point is that Barts enjoys higher perf/W than Cypress, so improvements were made in that area, and those improvements are likely to carry over to Cayman.
Is that really the case? From what I've seen the 5850 retains its leadership in perf/W over the 6800s: http://www.techpowerup.com/reviews/HIS/Radeon_HD_6850/29.html