I'll have to side with @
fatoni in that the FR-S chassis is at least structurally more rigid than the MR-S. One of the most common first mods for the MR-S is the infamous breastplate to mitigate lack of rigidity from being a convertible. Next steps would be FMB, RMB, MSMB, FSTB, and RSTB (common acronyms of braces littered throughout Spyderchat but not ft86club) plus roll bar to further stiffen the chassis. Basic coupe vs. convertible argument.
Typical MR-S aftermarket bracing:
The ring-shaped reinforcement frame along the FR-S roof and A/B/C pillars provides greater rigidity.
Both the MR-S and FR-S are monococque (unibody) chassis designs. But when we remove all body panels to expose the unibody skeleton, we can compare which is more intrinsically reinforced (FR-S rear quarter panel can't even be unbolted):
Lastly, MR-S side rail:
BRZ side rail:
The FR-S side rail is like a barricade of larger thickness that needs to be climbed over. This makes cabin ingress/egress more tub-like/Elise-esque than the MR-S. All this combined with the fixed roof make the FR-S almost an overkill exercise in chassis rigidity compared to the lightly reinforced MR-S.