I have been living with my SME 309 and external power supply for the last two months, and I am unbelievably happy. It took me a little time and a little help from some friends to get all of it 'right', but that was a lot easier than I feared.
Setup: The instructions from Sound HiFi are great. I will say that you need to read them pretty carefully (something I did not do on the first day). Following them carefully leads to very simple steps and a pretty painless set of mods.
Sound: I did both the 309 and the external power supply at the same time so I can't isolate the impact of each. I will say that the level clarity in the midrange that I have read Johnnyb53 speak to with simple insulation of the stock tonearm came through in spades. The midrange is astounding. The second biggest impact of the change to me was a lack of noise that was almost surprising. I did not previously have any power supply tweaks or any insulation on the tonearm so I cannot ascribe that to a single mod, but the whole set up is noticeably quieter.
The intangible: The new set up seems to have the weight or kick that I was looking for. The soundstage grew in depth, and the whole setup has a stability that is impressive (a characteristic of the 1200 already, I know). As some know, I was trying to find a new TT in the $3-5000 range. I ended up investing $1700 or so in the 1200 and am VERY happy.
Let me know if you have any questions.