In many environments, the extramemorylimit setting will have no effect due to php settings/available server memory etc.

the admin health tool has a class that checks this, (see problem_000008) - but it only displays a warning if the memory limit is lower than 128MB

It might be nice to do a similar check when displaying the extramemorylimit and make it read-only or force it to 0 if changing the setting will not do anything.