Not sure to what degree this has been tried yet as at the moment you are better off to run flat out? Approach will depend on which model of S7 you have.
162 Chip systems Batches 1,2,3,4,5,7 are a pure string design and you will just need to reduce the supply voltage to the Miner.
135 Chip systems batches 6,8,9,10 are a string design but powered by a Buck Converter, controlled by a PIC microcontroller and a Digital Pot. Although this has been discussed noone has yet fully understood or presented modifications to the Microcontroller code to allow the Core Voltage to be adjusted. I am certain this will be possible, but do not have an S7 to play with so cannot contribute. However if at some point 135 Chip S7 become affordable on the second had market I will almost certainly by one to play with.
Rich
That is the hard part about S7's to is all the different designs. Even the different frequencies and chip amounts it's hard to say what it will do in general as it's more individualized on which batch.
Look at the chart
That is a lot of variance.