As mentioned, the 'sandwich' 295 does have software voltage control, it's the only 200-series card that used a 55nm process that has it. The others (most of the 260 216's, 275, 285) do not.
This being said, because that card runs so hot to begin with, I wouldn't bother upping the voltage on air cooling. You aren't going to get much extra OC out of bumping it anyways. Certainly nowhere near enough to notice when gaming.
I had a sandwich model and iirc mine was one of the better OC'er vs. others on OCN. It did 675c/1242m (aka 2484 if you count the effect of ddr, which AB does) stable w/o touching volts. Most people were getting around 650/1199. I forget what my shader clocks were
Note also that 295's have distinct 'steps' when it comes to the clocks ... you can see it when you monitor the actual clocks in real-time on the AB graphs. So what you 'set' as your clocks ... may not end up being the actual clocks you're running at, they will default to the nearest valid 'step'. This is true w/all three clocks ... core, shader, and memory.
I wouldn't want either core going over about 95C, even though the actual danger zone is more like >100C.
There's lots of GPU OC'ing guides out there on teh webz so I'm not goign to sit around and re-write the proverbial book on that piece