"If you would find wearing noise blocking ear muffs, or sitting on a bouncy chair, or using a typing instrument instead of writing, improves your performance on a test, then yes that should be permitted."

The thing these examples have in common is that they don't give you any inherent advantage that invalidates the purpose of the test. (Assuming it's not a handwriting test, or an 'ignoring distractions' test.)

I would group all of these along with the examples I gave: corrective glasses, and wheelchairs. They should be available to all students, without diagnosis or discrimination.

If you think limited time on tests doesn't serve a useful purpose, then why give 'extra' time to only some students, and not to all students?