Definitely and interesting thought experiment.
On the nostr side I think each zap-like would produce 2 notes (kind 9734,9735 per NIP-57). Today likes (reactions) are a single note (kind 7, NIP-25). So if I’m interpreting the NIPs correctly we’d see double the notes if we moved to zap-likes as specified today.
On the lightning side I’d be surprised if there was much issue. We should be able to scale an invoice service backed by a database of small transactions to well over 5k ops/day.
What do you think?