Jason,
I don't know the answer but here is a data point.
Here is what one of the "best" Mustang vendors does:
http://www.maximummotorsports.com/instr ... 94v4r4.pdf
http://www.maximummotorsports.com/cc.asp
Here is what MM says about strength:
Q. What changed so that the MM Caster/Camber Plates now have a warranty?
A. For 1994+ cars, our lifetime warranty covers our newest Patented 4-bolt X 4-bolt configuration with the bearing plate on top of the main plate. This means that 4 bolts secure the bearing plate to evenly spread the load into the main plate. 4 bolts also secure the main plate to the car's strut tower to eliminate strut tower bending. The resulting structure is so strong, that we are able to offer the lifetime warranty.
There is a "bottom" plate for securing the studs and whatever strength and a main top plate. The instructions have some good pictures of OEM and MM's install.
The Mustang "stock" doesn't have the spring load into the shock tower since it is a "modified" McPherson strut. However, the design shown is also rated for use with coilovers (spring concentric with shock).