Medium-term effects of polyphasic sleep on performance
Disclaimer: As you have noted yourself, there aren't very many scientific researches on the topic. The only main points that I can derive are generally of blogs or sketchy speculations. As such, you are supposed to take this answer with a grain of salt.

Interesting Thing of the Day notes that polyphasic sleep may make the person awake and alert but have adverse effects on the creativity and mental agility. 


  ...it may be that although one can indeed be awake and alert for 22 hours per day, creativity and mental agility suffer as a result.


Supermemo has the following to say about Polyphasic sleep:


  Yet you will definitely not want to sleep polyphasically if:
  
  
  you want to maximize your creative output
  you want to maximize your peak alertness, your average alertness, or minimize the impact of your worst alertness levels
  you want to maximize the health effects of sleep, etc.
  


While both the sources are not scientific findings, it does send a signal that polyphasic sleep cycles would have a negative impact on IQ tests and general activities like reading because your overall alertness is down because of sleep deprivation. Poor mental agility would have result in a lower working memory as well.