The Intensity dial doesn't really work on Iray photometric lights and if you don't have 'Show Hidden' checked for the parameter option you won't even see it

Oops! Just checked and the Intensity dial is still visible with 'Show Hidden' checked in the latest Studio, however it doesn't seem to much impact on the light. It may change the fall off distant at rendering which would show in large scene. To truly increase the amount of light adjust the lumens.