Technically true if you are after the best possible stereo image, but in a car it matters zip !
There's so much other **** going on that's going to kill any hope a decent image like you would get from a home setup, that tweeters are generally mounted high up so you can actually hear them.
Woofers/mid-range need more air around them (and to move) hence why they generally sit lower down.
The human ear is very good at perceiving directionality for high frequencies. It's less good with lower frequencies. High frequencies are also absorbed more easily than low frequencies - hence why you can put a sub in the boot and still hear it.
Having the tweaker mounted higher up in the car "lifts" the sound stage and improves stereo imaging and frequency separation.
As NDS says, because of all the other noise, vibrations, acoustic inefficiencies etc within a car, you get less aural degradation by putting the mid range speakers further away from the ear where there is space for bigger cones than you do restricting cone size and movement in trying to position them closer to the ear.
As far as using co-axial speakers goes, if they're decent quality like the Infinitys, then yes, you'll get a big improvement over OEM and no, similar priced component speakers probably won't sound better.
If you can afford Infinity Kappas rather than the Reference series, then so much the better as they have more efficient cross-overs, better cones etc. I ran some co-axial 16cm Infinity Kappas in my previous WRX (with OEM tweaters disconnected) and they sounded great.
The component version would have been better but they were the best I could afford at the time and they were a lot better than the cheaper component speakers I could have afforded. In fact, I still have them and will be fitting them to my FXT as and when I get the time (MY03 FXT so no tweaters anyway).
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