I think we'll need to clarify a few things, with some scientific principles so we dont get ahead of ourselves!
Postion is important. How much? Very. Why? Because of the different responses in the near field, as opposed to the far field. Is it as important as pathlength differences? Absolutely. More? Probably. Why? Because pathlength differences only need to be ~30cm or closer in difference to achieve a singular coherent image from two wavefronts. Greater then that, and you'll start getting multiple images, or simply just no image at all because its too diffuse. With positioning, its actually quite critical.
Lets try to understand why.
Near field and far field are two distinctly seperate regions of space, that exist at certain distances away from the polar radiation of a driver. It is based primarily on frequency/wavelength. At 1kHz, its around 0.5 to 1 metre. Closer to a speaker then these distances, and your in the near field. Further away and your in the far field.
So what does this mean for me? Well, in the near field, the primary source of information is the direct wave. In fact, environmental responses have almost no effect at all. If you're clever (and you'll need to be really clever) you can manipulate the environment in the near field to benefit you in the far field. In this zone, intensity is almost equal everywhere. So getting closer to a speaker whilst its playing in the near field doesnt actually make it louder.
In the far field however, environmental responses override the direct wave as being the primary source of information cues. Reflections, refraction, resonance etc. all contribute quite greatly to our perception of sound. And intensity drops off quite quickly. Its 6dB per metre for anachoic conditions, or 3dB per metre for 4pi steradians (think a baffle).
Our tweeters are in the far field. The range of frequencies they play make this a given. In theory, we want BOTH the mids in the near-field. In practice (to achieve 30cm pathlength differences) they'll be in the far field. THEY BOTH NEED TO BE IN THE SAME FIELD TO ALLOW FOR SYMMETRICAL TUNING!!!! You CANNOT have one in the near field, and one in the far field, without a super-duper processor (note: none that powerful exists in Car Audio). Therefore, you MUST NOT mount them in your doors. Otherwise, you starting a 100metre race 200metres from the start line, and will never be able to get to the finish line in time.
The other thing to keep in mind is phase relationships, and sensitivity of the driver. Sensitivity (and the intensity changes when you crank the dial) is taken care of quite easily by matching the gain on your amplifiers (with a DMM/Osciliscope, impossible by ear). With phase relationships, if you have a tweeter playing 'midrange' frequencies (like below 5-6kHz Xover) it will have its own image, and therefore should be mounted within 1/4 wavelengths difference from the mid. This will allow it to be tuned so that you get a strong stable image, and not multiple ones.
If you use your midrange for midrange duties, and tweeters for higher frequencies (as you should in a 3-way), then you can mount your midrange down low, and the tweeter up on the dash and tune to allow the precendence effect to pull your stage up. Just make sure your tweeter is located either on equal pathlengths to the mid (same distance away) or further. Intensity is the key at such high frequencies, and time alignment will not help you.
One more thing. Before you buy a driver, study it. Find out what the cone is made of, and where the breakup nodes are located. A 4" peerless (like the HDS Nomex or Exclusive) use poly cones, and can play up to 4-5kHz easily without nasty breakup. The Seas can also play high, but its breakup is much more noticeable (from what I remember, its around 6-7k) and should be avoided (Xover lower) due to its stiffer cone.
Given the fact that all drivers experience beaming, you can get polar radiation right up to their beaming point (off axis and on axis has the same response). For ANY 4" mid, that means you'll easily push 5kHz without off-axis problems (domes go higher).
So in short, what does that all mean? 4" mids mounted in the kicks are the best, with tweeters in the kicks or the dash/pillars. Environmental response is a major consideration for these units (not the midbass anywhere near as much) and therefore start thinking about the environment MORE then what type of driver to use.
And for Seas drivers, try here....
http://www.madisound.com/catalog/index.php...88d349e92927637The Mid is $170 US each.