The Cone excursion graph will change as you change your input power. Whatever you have that set at, is what it will relate to. In my understanding, if it goes over excursion at 500w at a certain frequency, then any time that frequency hits 500w, it was have excursion problems. Of course, this really isn’t a problem assuming you are using a pro amp or dsp with a limiter. Just set your limiter accordingly and it will limit the signal power, preventing the sub from over-excursion.
As far as the size of the sub goes, I should mention if you want the shallower roll off with the 4 cubic foot just change the q of the filter to something like a 0.3. That should get you a similar roll-off. Having said that, all the detail we are working on are very minute and some of them might barely be audible (if audible). Either way, I am excited to see the build