165mm is a standard. So any 165mm speaker should fit unless it's too "long". But I don't think that will be a problem.
Better sound quality definitely. Stilo's Visteon-unit is 4x35W which should be enough for 165mm speakers if you don't except much bass sounds from them.
In my other car I have high quality 165mm's in front and 6x9"s in rear and the bass kicks better than my current Stilo system with subwoofer. And there's no amplifier, just the Sony head unit (claims 4x50W but 4x30 is probably closer to truth).
About connectors.. no RCA's, just some kind of special connectors. I haven't looked at the unit itself, just the separate sound manual that came with the car. It talks about line output connector for amplifier and/or subwoofer, but it doesn't make much sense. Probably bad translation. I see channel A and B line signals and subwoofer signals though. There's also a wakeup current for amplifier.
Someone else has probably taken a closer look at the back of the unit? Is every connector there, and is it possible to "steal" the line signals from them?
Stilo'04, Loden Green, 17" alloys, 8spkr "hifi" system, debadged.