I give some benchmark:
Only cryptonight heavy,blockchain driver, 1300core/2100mem, uber mix 3,1 (srb miner is much bether then cast or else):

rx470 4gb hynix                                800 h/r,
rx470 4gb samsung                           790 h/r,
rx580 4gb Powercolor samsung/hynix  817 h/r

only problem is primary card (attached dong on) with up to 4% low h/r (ON CAST AND CRYPTONIGHT HEAVY PRIMARI CARD IS 0 H/R)

My card have 4 GB(6 rigs with 6xrx470 and 3 rigs with 8rx580) Blockchain driver from august.On every rig is the same problem with only one-primary card. Hashrate of that card is around 100H/R 60-150 H/R but just primary gpu and thats happend only with Cryptonight heavy. Normaly I just reset miner and clock is ok but with Criptonight heavy allways is the same clock around 100h/s.

Cryptonight heavy have problem. I am on 6x rigs with rx 470 on every rig gpu 1 have low hash rate ~100 h/s. cryptonight v7 is ok