In my readings of various discussions on Warseer regarding what is "overpowered" it seems that a number of people feel that Slann Mage-Priests fall under the category of things that are overpowered. Personally I feel the only thing wrong with Slann as they currently exist has to do with Cupped Hands being able to bounce miscasts. I myself would just increase the points of Cupped Hands and then eliminate the miscast bounce and one use only. The other option in my mind is to give the Slann some other special rule or purchasable ability to limit miscasts. For example, a discipline that would only have the Slann IF and miscast on the role of three or more 6s.
I was wondering how others would like to see this addressed in the 8th edition Lizardmen army book and look forward to reading your thoughts on the matter.