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For sure there is a safety factor in place for any specified max loads. You will be able to carry loads of at least 50% more and be fairly certain you will not have any issues. There is just the small risk that if you do have an issue, you will not be granted any warranty. How much risk you want to take is entirely up to you.
In my eyes, the risk on a luggage racks is quite low. I don't think any carbon fork will catastrophically fail because of an extra 5kg, the worst that is likely to happen is a rivet pulling out or spinning.
It's funny what your told and not told.
I've seen people with pizza racks on their carbon forks with anything mounts. And people scared to run anything other than what the manufacturer says.
I wonder if all branded forks are worthy of carrying more than they let on..