Also remember that just because an OEM tweeter is mounted in the A-pillar with the midrange low in the door or kick panel doesn't mean that is where it will really sound the best. While the highest frequencies will seem to be high up above the dash, the overall sound will probably not be coherent across the audible frequency range, especially around the crossover frequency, and it will be difficult to make the system sound its best. Even if mounting the tweeter close to the midrange will place it low in the door or kick panel area, in most every case, it will sound better there with minimal effort, than if they are separated.
Sure, if you are using some serious processing power and have many hours to design the system, you can make it sound really good with the speakers separated. But we're talking about making systems sound good without extensive vehicular modifications, thousands of dollars and many hours in the install bay.
Once the system is tweaked with the midrange and tweeter close together, if there is the desire to raise the overall soundstage, a second set of tweeters can be used, carefully mounted higher in the A-pillars and crossed over very high (above 10,000 Hz). Done carefully, this can help with the apparent stage height, without detracting from sound quality of the primary midrange/tweeter system.
Conclusion
These are just a few techniques, tips and guidelines that can help make your system sound its very best. While it may not be possible to accomplish all of these things in every installation, the more effort that can be put into the installation, the better the system will likely sound.
We all know that subwoofers are really important and help provide the low frequency foundation in any system. But, the speakers reproducing the majority of the music in the range above the sub-bass range are what will truly determine how the system will really sound. Design and install the higher frequency speakers the right way, and the system will have the potential to sound awesome. Install them poorly, and there will be no possible way to make the system sound good.