You could probably pinch a couple of mm by offsetting the EBB shell.
Out of interest, what are you hoping to gain? Do Thorn offer any external BBs on any model? I'm just wondering if the EBB insert is ready to take the cups or if it needs facing the way most new frames do.
I've stuck with square taper, from road bike to touring, I'm not adverse to change, they've always served me well and I can't see any advantage.