Hi soy,
If you really looking to improve efficiency then you have to down-clock the S3, the sweet spot is 150Mhz.
Once you drop the clock you should be able to lower the voltage even further down than 9v on your power supply.
I like the idea of simply moding the power-supply instead of moding each of the 16 step down voltage converters on the s3 board, which is a major p.i.t.a.
I would really like to see your results at 150Mhz and see if your power supply mod can match my S3 pencil mod efficency (180w @ 305 Gh/s on the wall)

Regards,
-Jay