if it's anything like the mk2 speakers with sub it'll be better than standard but if you're after vastly improved sound quality then wouldn't bother with it.
Yeah a 100 watt sub will be like about 40w RMS into a crappy paper sub and i bet it cost more than a decent system that you can buy and have fitted pro.
shouldn't do. I fitted mine whilst still under warrenty and actually had to take it to fiat to be looked at and it wasn't an issue. you won't be doing anything which would affect electronics or anything like that as you'll be taking your own power feed from the battery and putting in your own wiring.
just out of interest what are they charging for it and does it include fitting?