OK.. To put it as concisely as possible, the main area of weakness is the transition from the neck to headstock.
As you are aware, a grubson neck to headstock transition is rather flat and sudden.
The simple soloution to strengthen this is to make a triangular transition, ie, leave a bit of meat there.
This wont affect the playing comfort at all and would make the headstock a heck of a lot stronger.
The other way would be to machine the neck out of a single chunk of tree, thus doing away with the join.