Larger battery will drill more holes but wont make it drill quicker.
Not sure but if has different speeds and modes (hammer, drill, screw) you can adjust that and will help a bit. Want drill mode and slow to moderate speed.
6 drill will be significantly quicker than 8 depend on brand and blade conditions.
6 will work for anything but the pike and musky. Pulled a 8lb walleye through a 5 hole, just a bit more difficult than 8.