Adding heatsinks help but now you're making visible changes.
J4bberwock & nst6563 would know for sure if a "pencil mod" would be beneficial or too risky. Maybe it could help reduce HW errors at slight overclock speeds. The results would not be precise though. If you reduce the resistance too much the voltage will get too high and you could see smoke There's no way to put the smoke back in once you let it out.
The volt mod so far is replacing one small resistor. I actually paralleled a resistor using what I had on hand. You need a steady hand and a very fine tip on a soldering iron. Cooling comes first. They're mini heaters.
Definately do the cooling mods. I was able to go from 333 to 350Mhz with just that alone.
As for a 'pencil mod'....I suppose you could do it across the resistor that we're replacing. The problem is that it's not a decent reliable method. You would have to draw a line, plug in and test the voltage point, then redo the process until you got to the voltage you're looking for. Total pain in the ass in my opinion...if you're experienced in soldering, it would actually take you less time to just swap the resistor than to attempt a functional pencil mod. I replaced mine with a trim pot so I could make small adjustments without having to swap out resistors again.