The AMD Nano/Fury line are all MUCH higher core count, faster clocks, faster memory, MUCH wider memory bus - and mine SLOWER than my 2-generation old R9 290s, which routinely see 30 MH and I've not pushed them as far as they CAN go (best figure I've seen for the Nano was 27).
There appears to be more than just pure "memory bandwidth" involved in how fast Ethereum can be mined by a given model of GPU.