1. If the system is stable, no.
2. The only real gain is lower system temps.
3. It's the other way around. After a certain point, overclocking requires overvolting.
4. CPU speed 25% faster. Overall system performance will be less.
5. No way of knowing without trying. Only thing for certain is that higher ambient temps will give you higher system temps. That is why monitoring temps while overclocking is so critical.
6. You probably will not see much difference - same GPU combined with either competent CPU.