I run an s5 for fun but the s7 would cost me 250usd a month to run (in loss)
I guess I have to wait for he BF chips, if they ever make it to the open market
I'm going to simplify your concerns. You can't make it cost effective. The end.
You could try using a PSU that would feed undervolted wattage. Underclocking would not increase efficiency. Undervolting would, but there's not much information at all on how low we're able to bring the S7 down to.
Even if you could do it nicely, it would not be cost effective for the hash AND even if you brought down the S7 magically to .2, you would still not be able to profit.
Your only hope to profit would be hosted solution at 0.05$/kWh or less, right now with the poor profitability of S7s.