No, not that it won't tack. but I'm more interested in dimensions. I have two sails, both P&B on has a batten labelled extrastiff, which is 15mm wide by about 10 thick, the other is maybe 12mmx8mm (ie obviously less stiff). Apart from the tacking issue there's the shape of the sail to consider. What do other people use?
Crew says she thinks extrastiff sounds best, but what would she know?

I think the buckling load should be in range 2.6 Kg to 3.0 Kg.
To measure this, stand batten vertically on kitchen or bathroom scales and press down with increasing force with palm of hand on upper end. The batten will suddenly buckle at a fairly clearly defined load. The two battens will give very different loads - the fatter one is 2.5 times stiffer than the other, assuming they are both made of the same material.
I think the battens should be uniform in stiffness along length, i.e. no tapering.