I don't have power meter on my own but what did your mate found out on your setup?
I HAVE DRAWN THE CONCLUSION IT IS NOT WORTH UNDER-CLOCKING THE S3 or S3+ [At least at present price of bitcoin]. In fact, the more over-clocked they are the better. I don't know that it would ever be worth under clocking it? I can see under-clocking to make more power available to add more rigs with a higher hash rate to your present set up. That's about the only advantage. As well as having a faster interface with the new firmware upgrade.
S3 @ 270 watts [under-clocked at 175 MHz @ 0.68 volts (0680) and 340 to 360 GH/s (I chose 350 GH/s for the math)]$0.106 (10.6 cents) per kWH power costs for 270 watts:Cost Per Hour: $0.028620
Cost Per Day: $0.686880Cost Per Week: $4.808
Cost Per Month: $19.23
Cost Per Year: $250.02
Power costs at $0.106 (10.6 cents) per kWH for 380 watts:Cost Per Hour: $0.040280
Cost Per Day: $0.966720
Cost Per Week: $6.767
Cost Per Month: $27.07
Cost Per Year: $351.89
With BTC @ $273.471 & Difficulty at 46,684,376,316 and S3 tuned to 175 MHz @ 0.68 volts for 350 GH/s @ 270 watts:Mined Per Hour: 0.0001571 BTC $0.04296 USD - $0.028620 pwr. costs = $0.01434 per hour
Mined Per Day: 0.00377 BTC $1.031 USD - $0.686880 pwr. costs = $0.34412 per day
Mined Per Week: 0.02639 BTC $7.217 USD - $4.808 pwr. costs = $2.409 per week
Mined Per Month: 0.1131 BTC $30.93 USD - $19.23 pwr. costs = $11.70 per month
With BTC @ $273.471 & Difficulty at 46,684,376,316 and S3 tuned to 237.5MHz @ 0.725 volts for 480 GH/s @ 380 watts:Mined Per Hour: 0.0002155 BTC $0.05893 USD - $0.040280 pwr. costs = $0.01865 per hour
Mined Per Day: 0.005171 BTC $1.414 USD - $0.966720 pwr. costs = $0.44728 per day
Mined Per Week: 0.03619 BTC $9.897 USD - $6.767 pwr. costs = $3.13 per week
Mined Per Month: 0.1551 BTC $42.42 USD - $27.07 pwr. costs = $15.35 per month
The following screen shot shows what is mined in BTC with 350 GH/s:The following screen shot shows what is mined in BTC with 480 GH/s: