Official comment

Hi Rudolph,
you're right when saying that importing 1000 items will trigger 1000 webhooks, but we don't impose any rate-limit on webhooks.
In the use-case you depicted, you will "spend"
1 call for exporting
n calls for updating (depending on how many updates you do with the API)
1 call for importing
Hope that helps.
Best,
Domenico