Been using Windows 8 every day for just over 4 months now and I've not noticed any difference in gaming between 7 and 8. Performance is pretty much the same really (for me, at least).
Just use whichever you prefer.
Depends on your motherboard and what kind of voltages are using as to whether your cooler will 'handle it' or not. Make sure you monitor the temperatures at idle and at load.
As for gaining performance, you might see a marginal increase but probably nothing spectacular in gaming. You may notice a bigger increase if you do stuff like rendering or encoding though.
4.4GHz is a pretty respectable overclock, but if you can do it safely, by all means try and up it.
Try to keep the voltage as low as possible and the multiplier as high as possible. That way, you can achieve the highest overclocks but with the lowest amount of heat (hopefully).
However if you use too low a voltage, you'll get instability issues (crashes, blue screens or reboots) and if you use too high a voltage, you'll be running hotter and possibly get instability issues due to heat.
Oh ok... so this is like a trial and error so to speak?
Pretty much unfortunately.
Record each setting you try (write it down on a piece of paper or something), and then if you get something which works, even if you later muck it up by fiddling, you can easily go back to it.
Some BIOSes allow you to save your overclock settings in different profiles. Pretty sure my Z68 board (which is now two generations old, sad realisation of the day) allows you to do this, so your Z87 probably also supports this.