I would think it might be much lower.
Standard NiB Magnets have a maximum operating temp of
176 degrees F(80 C) and a max curie temp of 590 degrees F(310 C)

The max operating temp for drives is around 130 degrees F.

So if the magnets start degrading at 176 , I would think the
media would degrade before magnets.

And then of course there is also the warpage factor

Just my humble opinion