Nope.
I found that as not true.

Take the Colorful 8600GT OC Edition for example, it's pre-overclocked to 580MHz Core & 800MHz Mem.
But, i manage to overclock it to 650MHz Core & 1000MHz Mem.
Stable all the way to benchmarking and playing games for hours.

The same goes to my BFG 8800 Ultra Overclocked too.
It's pre overclocked, but i manage to overclocke more than what is set by the manufacturer.

What are you trying to say?

Yes, but the Nvidia driver will set both of your graphic card to be the same as either one that has lower speed.

Let's say first graphic card is on 650MHz core clock while the 2nd one is on 550MHz core clock, once you're on SLi, the Nvidia driver will automatically set the the first graphic card to follow the core clock of the lower clock speed that is the 2nd graphic card.

Nope.
Version maybe which is Overclocked Edition, Ultra Edition or some other edition, but i never heard of GC have reivision, stepping

You want a GC that is easier to overclocked, get those that has been factory Pre-overclocked.

.......
Offcourse can ler.

Doing VMOD.

As long as there's no parts on the card that fall from the card itself and there's no burntmark, you card is still valid for RMA.

Owh, and not to forget to make sure that you never leave any traces that you've opened the heatsink even on the original screws.