that comment is not true at all,you dont have to amp them but it will obviously benefit.your advantage of comps is that you havent got one speaker trying to do two jobs,components allow the two speakers to concentrate on what they do best
apart from a coaxial has two parts to it too. Both the tweeter and midbass driver so that comment is not totally true either.
It is impossible to say which is better. In some cars a coaxial will work better and in others a component system will work better.
U need to look at where the speakers are mounted etc and with knowledge from previous installations i would then make the decision as to what would work best.
As ive said before, putting the tweeter in the original position on a clio is not the best of ideas because u will suffer from reflections from the glass and putting the midbass and tweeter together (close to each other) provides much more cohearent sound.
When the tweeter is high up it CAN also sound more dominant and often sound too bright and aggressive.
In some big installations, yes the tweeter could work well on an A pillar mount but then we are talking about much higher quality tweeters which dont sound nasty and dont take your head off!
Lets put it this way, in my old 182 i used to use JL Audio C2 coaxials in the doors of it and most were adament that i had tweeters on the dash!!
I also would consider doing the small modifications to get a 17cm speaker in the door as u will gain almost double the cone area and will sound significantly better and offer u much better power handling and more midbass