I have 10 rx 470 4Gb nitro+ sapphire. Some of them have bad basic quality (60%, better than 1.5% of similar GPU's) and they run at 29.5Mhash stable because they have samsung memory.
So i would say that the real problem in my GPU's is the memory, GPU's with Heriox memory are shitty and i cannot run the higher than 27.5Mhash even though they have high ASIC quality.