Yes it has a default of 340 and max of 451 so 150%. As for the voltage, you don't need to restrict yourself for 'daily' settings vs benchmark settings unless you're an ln2 user going above 1.4v. You can safely run your card with 1.35 volts without any damage of any sort. You'll be throttled because of a power limit and that's it. Safe card voltages are usually more applicable all 770's or all 780's rather than just one card in particular, but of course high end cards like 770 Lightning will take a lot more voltage and stay stable anyway.
So it is completely fine if you run with the AB slider at +100mv all the time and have your card at 1.313v by default like mine, in fact I recommend it. Add as much voltage as you can on stock clocks until it starts hitting the power limit too much then back it down. Now you have your max voltage and can overclock until you get glitches with and back that off 13 Mhz or something and you're good.
Your bios will probably come with 1.187v so you'll get 1.287v with AB and can add some more on with the hack if you want.
As far as bios flashing, don't worry about that. It's not as dangerous as it sounds. Your biggest threat is having an unstable system or power grid which causes your system to fail in the middle of the flash, which is like 10 seconds by the way. If a bios is modded improperly then there is another risk. You can always easily recover though if you have anything else to see your display with because you can just try to flash again while running on onboard graphics or some old AMD 4350 low profile thing that came in a computer from walmart.
Edit: Galaxy 770 HOF is only 20$ more and is binned. I'd definitely go with that one. Ration your dinner if you have to, I would. Edited by cookieboyeli - 11/1/13 at 4:56pm