That chart seams to be purely theoretical, or may been results of the chip design simulation.

I did a whole batch of undervolting my S3 to different voltages trying to replicate the efficenty in the chart.
At 0.72v the max clock could not be higher than 193Mhz and resulted in 380GH/s at 280w on the wall. That's only 20w improvement over the unmodified voltage ( 300w at 380GH/s )
The gain by undervolting is not comparable to the S1 where you could get almost twice the efficency with a careful pencil mod.
Although for me it was still worth it, because of my power restrictions and heat problems in my small apartment.
Without intend trying to keep the max GH/s In mind I found my sweet spot at 150MHz clock rate and 0.68-0.69v producing 310GH/s at 180w on the wall.
Also I really wanted to get the consumption per S3 below 200w to be able to use a single PCIE power cable per unit so I can have all 6x S3's running of only one 1500w SilverStone PSU.
The end result is 1.8-1.9 TH/s at 1100w on the wall (80plus Gold PSU) which is still only 0.6w per GH.
So that may be one of the reasons why ther is no "official" S3 pencil mod. It's simply not worth the trouble.
In my case it was more about the power restriction and heat output. Which is now more like a warm breeze on the fan exhaust rather than standing in front of a space heater on max in my small Texas heat 625 sqft Apartment. As nice side effect the fan noise is a lot less now (from 2100-2400 rpm Down to 1000 rpm). With some F12 fan replacements I could make the S3 completely silent now.
The mod is a big hassle and very tricky with the having to remove the case and boards. The location of the resistors ( on the inside/back of the board ) is also tricky to work with.
And the PCB seams to be very fragile, I managed to kil two boards. I barly touched the board and yanked off a tiny capacitor this was sticking out from the board. After carefull soldering I tought I fixed it but must have shorted something and resulted in a dead board. Even only one bad voltage regulator ( 1 of 8 per board ) results on a completely dead board.

I made plenty of pictures during the mod, but I feel the gain in efficency and General PITA to do the mod is not worth the write for a tutorial.

-Jay