Taking an eyes wide open look...
Failure in a part that can probably be attributed to construction/installation factors, will not necessarily be resolved by simply making that part stronger.
In fact, there is a good possibility that if the same type of installation error occurs when using a stiffer/stronger part, that it will simple move the failure point somewhere else (to the fwd vertical stab. spar for instance).
Considering the number of RV's (pretty much all of them), flying with this type of vertical stab installation, factored against the small number of attach brackets with cracks, it seems the correct approach would be to be sure that the bracket is properly deburred, and not under any strain when installed.