Saltfever,
You seem to be convinced that because the rollbar baseplate “punched through” the floor pan that it was a shear failure. However, to have a failure in shear there must be two opposing forces in close proximity acting across the material section to produce shearing stress. How many scissors have you seen with only one blade?
The floor pan failure is substantially a tension failure, but the failure is very localized, namely around the perimeter of the baseplate where the floor pan is being loaded by the edge of the baseplate. Load from the baseplate causes the pan to sag and develop tensile stress in the pan material, since the outer edge of the pan is presumably restrained from moving. As the load increases, the sag increases, and the tensile stress increases until the tensile strength is exceeded. This occurs at the edge of the baseplate because that hard edge creates something of a stress concentration there due to the bending of the pan around the corner, and also because away from the baseplate the pan has more material to carry the load, and the tensile stress is therefore less in those areas.
So, given this failure mode, it is apparent that the bigger the baseplate the more the load that can be carried--which is the reason for having a baseplate in the first place. Eliminating the stress concentration caused by the hard edge of the baseplate, by tapering or rounding the edge or using a thinner baseplate may make a small improvement in the load capacity, but probably not much*. And those actions may reduce the ultimate capacity if they, in effect, shrink the effective perimeter of the plate and cause even less of the pan material to be carrying the load. Those actions to reduce the stress concentration may, however, result in marginally greater energy absorption before failure since more material is being stressed.
*Because, once the material with the highest stress in the cross-sectional thickness reaches its yield point it begins to stretch, passing any increased load onto the region next to it until the whole cross-section has yielded and the whole section then stretches until it reaches its ultimate strength.
