While your theory holds merit on paper, in the real world its not exactly true. It is true a sealed enclosure is an air cushion, affecting cone movement. This does affect a cones ability to reproduce specific frequencies, the cone still moves so many times per second (to produce the wave).
What you need to consider with a sealed enclosure is, its always wanting to return the cone to its center position, due to changes in air pressure. Certainly this is a 'restriction' on cone movement, but you only see this as hurting the speaker's performance, that's not true. For example, remove a speaker from its sealed enclosure and run it free air (true free air, no baffle or anything) and let us know how well the cone control works out. =P
Vnted, or ported enclosures also put restriction on the cone movement, surely you didnt think otherwise? Its true a ported enclosure has a hole in it to dissi[ate the pressure changes, but it certainly doesn't do it instantly. The cone pushes and pulls that air through those ports, the cone is not free to move without restriction.
Now, if you want to talk SQ, you need to keep in mind its all theory. Anyone have a meter that measures 'SQ'? //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif So lets look at a vented snclosure for a second, see how one is constructed. A vented enclosure is 'tuned' to a specific frequency. Right there that should tell you a vented enclosure will not have a linear output along its entire frequency range, which is inheritantly parallel to 'accurately' reproducing sound. In other words, there is no perfect enclosure.
But, and this is a big but, you have to figure in more than just the speaker/enclosure to determine overall SQ, right? Of course, what it sounds like inside the vehicle affects overall SQ. What Im getting at is, a car has a resonant frequency, each car is different. This mean, the size and shape of the vehicle, and even the materials its made of, can affect how it affects the sound inside the car. It can 'absorb' some frequencies and 'amplify' others. Your linear response on paper from that perfect speaker/enclosure goes right out the window. True SQ competitors know this, and factor it into building/tweaking their systems. If your car resonates at 50hz for example, you aren't going to get good SQ from a ported enclosure turned to or close to 50hz. For example, if a person were to find their car absorbs sound around 30hz, a savvy competitor could/would build a box tuned to that freq to adjust, then smooth it out with an eq.
Now, back to a sealed enclosure. The reason sealed is generally considered best for SQ is because its restrictions on the cone (and freq response) are linear. It doesn't amplify any specific frequencies, because there is nothing to 'tune' when building it. Its output is pretty much linear across its entire frequency range. But, as Ive said above, making a blanket statement like sealed enclosures always do better for SQ is misleading. But, saying a ported enclosure is better is just as misguided (and moreso imho).
If you want to have a serious debate about SQ, you cant stop at the enclosure and speaker. Subs dont play in a vacuum (although they do draw a bit of one in a sealed enclosure =P).