What are the design limits of a normal tub overflow drains (and their gaskets)?
Wow, he might be right... according to this, &quot;overflow drains&quot; are a complete misnomer and the whole assembly is YMMV/at your own risk/a farce:

According to PMI (Plumbing Manufacturers Institute) no manufacturer currently makes bathtub overflows, only trip lever assemblies are made.
...
If you disagree then I challenge you to come up with the documentation that these so called overflows are a real plumbing product in the same sense as any other plumbing product being what it claims to be:
• Show that a standards specification exists with ASME, ANSI, NSF, IAPMO, or any other approved standards making group.
• Show the documentation for the engineering calculations that specify the flow rates the overflow should be capable of handling and how this relates to the overflow &quot;protecting against accidental flooding resulting from an unattended filling operation&quot;.
• Show what listing agency is providing a listing for such a product and what testing protocol they are using to verify that the overflow is meeting the standard.
• Show what manufacturer makes an overflow that has such a listing and stands behind the product with a warranty that the product will &quot;protect against accidental flooding resulting from an unattended filling operation&quot; according to the standard to which it is listed.
You will find that none of this exists.

Yikes.