If you look under the fairing near the vertical fin you will see that the mounting bracket for said fin is only slightly thicker than bacofoil!
Waldo, I'm not sure I understand this. The fin attachment on the -4 and -8 are similar. The rear fin spar takes the direct rudder loads and is bolted to the fuselage through a 1"*1"x1/8"thick bracket made of extruded 6061 alloy (same material and section as the longerons).
The front spar, which I think you may be referring to, attaches to the fuselage via the tailplane forward spar through a .063" thick 6061 alloy plate on the tailplane and similar 1"x1"x1/8" extruded brackets to the fuselage.
While building, I've always been impressed by the design work that has gone into the RVs, they are very strong, generally elegantly designed, and well up to the g-limits and Vmc that Van specifies.
It is designed to take full rudder deflection at over 120 knots, stall turns and aggressive side-slips, all of which are likely to load that area more than a flick-roll.
Have you compared it with any similar aircraft?