you're actually noticing 2 variables at work.
one, generally speaking, 8 ohms speakers have a sensitivity similar- or greater-than a 4 ohm speaker so it takes less power to get them movin.
example of a sub i'm looking at:
8 ohm 2.83V / 1m 87.2 dB
4 ohm 2.83V / 1m 88.4 dB
not bad, right? half the power, only 1dB in loss (you can't hear that
). they're both rated at 175 RMS, BUT the 8 ohm will be more efficient, reducing the strain (load) on the amp. my amp is rated at 400 RMS @ 4 ohms, giving it the perfect power at 8.
and two, the use of pro audio drivers. i'll guide you to
this thread, and give you the tl;dr cliff notes.
basically, when you use pro audio speakers in the car, you stick them in the smallest box possible and give them ridiculous amounts of power. due to their design and love of abuse, they can take the combo of too-small-box-and-too-much-power that would destroy 99% of "consumer" speakers.
in the DIYMA thread, patrick is trying to duplicate the insane amount of up-front midbass an old school competitor's car had. his secret was using a pro audio 12" in the rear side panel. i highly recommend reading the thread...patricks threads are always great
as for the last part of your question...8 ohms are just the "standard" for 120V. wiring isn't any different, but you need 8 ohm drivers and amps that can run at 8 ohms (most that are "only" 4 ohm stable can also do 8). DO NOT run different impedences on passive crossovers.