Does it matter where the weight sits on a cantilever shelf?

Yes in practical terms, because a cantilever is a lever, not sorcery.

We have a published 15kg maximum load but no separate manufacturer figure for a load concentrated at the extreme rear edge.

I would keep a substantial item stably supported with its centre of gravity reasonably close to the rack rails rather than deliberately hanging the bulk of the weight as far back as possible.

If the installation approaches the load limit and is markedly rear-heavy, a deeper or rear-supported arrangement is the more convincing solution.