I ran mine at 700MHz and corresponding shader (synched) for a while. But backed it down. I felt it was fast enough already and the o/c was just for curiosity. So, mine had at least 100MHz of headroom. But you can't go by mine. Yours may be better or worse. That is a given.
I had an eVGA GTX 280 SSC that would overclock pretty good. My friend has an eVGA GTX 280 SC that just refuses to overclock at all without some sort of stability problem. If I remember right I think mine would kick out 702MHz(stock) but after the vmod it ran fine @ 792MHz. So basically just about any 280 should be able to hit around 690- low 700's.
The pair of 4850's I had were insane. Not too much to write home about on stock vgpu but a vmod and water cooling really opened those cards up... 950MHz :Q That's a 325MHz overclock on a gpu. I'd still have them if they were a little better on the memory bandwidth side of things.
Nah, I don't mess around with pencil mods. If you invest in a decent soldering iron it's not much of a risk if your soldering skills are up to the task and you have good cooling. I wouldn't really mess with a vmod on a 280. If you're getting in the 720's (I think that's what mine was running on stock vgpu actually...not 702MHz) it's probably not worth a vmod. You'll have to unlink the shaders to get close to 792MHz and the performance jump and cost of extra cooling(sounds like your good here) isn't really worth it. On the other hand when the 55nm GT200's show up I'll think a vmod'd one would be an interesting thing to see.
Vmodding everything is a sickness :evil: that some people have. If you have to ask if it's worth it then don't do it.