Is the 470 a different die or something? Doesn't make much sense targeting a fused off chip for perf/watt numbers.
AFAIK the 470 is a cut 480 die (2048 vs 2304 SPs), I expect the perf/w savings will be primarily from targeting lower clocks. I know there's been some rather pessimistic views on the perf/w of Polaris based on the 480 but I personally expect the 480 will be the outlier rather than the norm. Here is the reddit post in question, my takeaway is that we will be seeing Polaris cards with very competitive perf/w, just the 480 isn't the best at it.
Some good news: The Sapphire Nitro 480 has been negotiated down to £249 in the UK. Reference 8GB 480s are around the £220 mark. £30 extra for better cooling, power delivery and overclocking? Yes please!