I got my 2 Sapphire 470 Nitro+ 4GB with Samsung rock solid @ 29.5MH/s on eth.
This is on Linux box... Maybe win would be a bit faster.
I got 30+ but it's not that stable...
Can you post the strap you are using? I'm not having much luck getting my Sapphire 470 over 27Mh. That's running memory at 1900 with the 1500 strap. Using the 1375 strap gives me errors mining ZEC.
Here's the 1375 & 1500 straps from the stock BIOS:
333000000000000022CC1C008CCD593AC0570D13AA09B4070048C7007A0014207A8900A002000000140D272E7B252B15
555000000000000022CC1C00AD515A3EC0570E142D4A64080048C700030114207A8900A003000000150E2A3186272E16
Use 1625 Timing it's much better for ethereum.