This depends—even if the diadem really does make you smarter, it is entirely possible that Rovena exhausted all the low-hanging fruit available for using magic to improve cognition -
For example: If what the diadem does is use healing and vertiaserum type effects to keep your brain in peak working condition (ideal blood sugar, magically eliminating waste toxins, ect, ect) and force you to be intellectually honest, that would allow you to think as quickly and as well as you do at your best all the time - which would be a huge upgrade—but it would not allow recursive improvements.
This depends—even if the diadem really does make you smarter, it is entirely possible that Rovena exhausted all the low-hanging fruit available for using magic to improve cognition - For example: If what the diadem does is use healing and vertiaserum type effects to keep your brain in peak working condition (ideal blood sugar, magically eliminating waste toxins, ect, ect) and force you to be intellectually honest, that would allow you to think as quickly and as well as you do at your best all the time - which would be a huge upgrade—but it would not allow recursive improvements.