As the cryptocurency (Bitecoin, Litecoin…) miners quickly realize, the AMD graphic cards FP64 double precison give them very powerfull edge on mining. And that was when officialy the double precison is 1/8 speed of the single precision calculations for R9 GPUs. In reality, most of the GCN architecture can get in real life to 1/16, 1/4 and even 1/2 of the single precision speeds.
However people argued, that Hawaii chips (R9) should not support 1/2 rate od double precision over single precision, because Hawaii chips are 438mm2 in size, while Tahiti is 365mm2, witch is then only 20% larger AND it come with 38% more of stream processors, 50% more ROP units and 33% wider memory bus.
It does not make sense (even that the memory interface get surprisingly smaller when getting wider in Hawaii), but ... AMD now claiming it:
(looking at the stacked cards I find hard to believe that this system could actually work - that is, because there is almost no place for air intake in the top 3 cards that could the blower use… not to mention that AMD cards from the R7/R9 GPU family produce too much heat and the professional versions of AMD cards did not use any different GPUs, so the problems with cooling will remain and in fact, by stacking the cards like this will probably result in catastrophic thermal failure... but hey, at least John could hook up now SIX 4k monitors there )