I have the 24" high top on my GMC Savana.
The finished interior height is 6'2" after I finished the ceiling and the floor in the center.
The floor is 3/4'" plywood plus the depth of vinyl laminate so app. 1" total. The ceiling is somewhat vaulted because I glued 1x2 strips of wood (3/4" thick) to the fiberglass high top and then added Luan panels that are covered with 1/8" closed cell foam sheeting and marine vinyl. Overall I lost just under 2" from metal floor to the raw fiberglass.
The floor to ceiling height in an E-Series will vary depending on the year of the van but only by an inch or two.
Wind resistance is one thing, roll-over is another. Yes, there is wind resistance with the high top but not extremely different from a regular van height. By the time you've got a cross wind on the van the broad side of the van is not increased greatly by the high top.
Roll-over depends on the center of gravity. Yes, it's higher in a van than in a car but as long as you've got most of the weight at the floor level, adding the high top does nothing to move the center of gravity. Now, if you put 600 lbs of batteries up high in the high top, THEN you're moving the center of gravity upwards.
The history of roll overs in vans (and the fear) stems from incidences of extended length 15 passenger vans being used to transport adults and other heavy loads. The suspension system becomes overloaded, particularly in the rear, thereby lifting the front end and making it unstable. You're not likely to be adding the weight of 15 adult passengers (probably in excess of 3,000 lbs and even if you do it's going to be on the floor level not up at seating height.
BTW, my Savana with the 24" high top and a Camco roof vent cover tops out at about 10' 6"...not a problem on all but the lowest of bridges. And of course, drive-throughs...
