It has been shown that one can make a decent conical air column by soldering together a bunch of cylindrical sections in increasing diameters; the price is only a certain amount of acoustic efficiency. Further, many saxes have the end of the neck cylindrical rather than conical for the last inch or so, and somewhat narrow (necking in), and that actually improves the purity of the high notes. Further still, the air column is disturbed by a huge number of tone hole chimneys, which certainly disturb the conicity. Not to mention the fact that the whole tip of the cone is missing, replaced by a blob of mouthpiece. And yet the horn plays, wonder of wonders...
The actual change in bore volume along the tenon is miniscule when compared the the overall bore volume. You have a much larger perturbation at each tone hole chimney. As Joe Wolfe from UNSW pointed out to me in a private correspondence, it would be easy and inexpensive for manufacturers to fit a conical tenon, and if it really improved the horn acoustically, they would have done so long ago to ace the competition.
Likewise the neck constriction. My tárogatós are conical, with the basically same bore profile as soprano saxes, but with mpcs that are more like a clarinet, which fit all the way down into the body and have no step. It's not like there is any great difference based on that. Nor is there any great change in behavior or intonation when the mpc if pulled out slightly for tuning purposes, leaving a rather large gap between mpc and body. At one point I fitted a cork shim inside to fill the gap, and if there was any difference it was extremely minor--nothing that I noticed in normal playing.
Yes, everything in the air column matters, the question is: how much?