Thank you to everybody for your contributions to the debate. The message from jornymn is quite elusive to me. In Europe all the battens come from the same extrusion. This means that, apart from lenghts, they are all the same non larger or smaller. This means that their load is incresing from bottom to top because the profile is the same but the lenght is different. The load increasing from one batten to the other is resonably even, apart the one relative to the number one top batten that is far too much stiff.
Summing up all your advices my actual thinking is that is better to avoid tapering the battens, above all if they are used with an old sail, except from the top one that must be treated quite intensively.