A "pencil-trick volt mod on a 8500 is..............well, tricky. The problem with it is getting just the right amount of lead to lower the resistence properly. All the guides I have seen suggest doing this with the card out of the machine.
But, In my opinion, while that may be safer in one respect, it is very dangerous in another. You have no idea how many strokes, where, ect.ect...and risk a bad overvolt/dead core.
So, I did the pencil-trick with the card in the machine powered up. Yes, this is risky also, but with a VM, you can be very accurate and set the voltage exactly where you want it.
Even so, I killed my best 8500 doing this
Pencil slipped a little and well, thats all it takes.
I replaced that card with another 8500(Sapphire). Seemed like a really good card at first 3.3 Hynix where my retail BBA had 3.6 Hynix. Thing is, I could crank the memory to 355, but the core would do nothing after the pencil VM. So I decide to do the full blow XBIT LABS VM. Only I used vr's instead of fixed resistors for tweaking. Well, this card just would not cooperate. So even if you do the VM, beware................it might not make much difference. That fact is summed up by the XBit Labs article from the beginning. I really thought that VM would have done something as the pencil mod only changes two voltage points, wheras the Full mod makes 5 voltage changes.
So to answer your questions,
-a pencil volt mod is not an easy thing to pull off correctly.
-You might get a 10-15% overclock out of it, but it's not a sure thing.