Therefore, the question remains, is there any difference with regards to rigidity between pop-top and hi-top?
No, none because the A, B, C and D pillars are all in place with their respective roll bars and gutter bars.. the hole for High top use is usualy between the B and C pillar, as is the pop top though Devon and Diamond RV use another smaller hole, for entry to the top when the bed is down.
fiber glass will offer nothing to the vans intrinsic strength.
The only high top to remove the C pillar roll bar, are Westfailure high tops, it is replaced with their own version that runs over the roof.