The softener plays a major role in removing iron. I've tested if both ways with and without the prefilter. I installed the pre-filter mainly to reduce the work of the softener and prolong the softener resin life.

Before the pool forum data was lost, Ben had an extensive post on using a water softener to remove metals that included Iron and Maganese.

For my pool it worked wonders. I did go thru a lot of regeneration cycles and salt for a 26000 gallon pool. I'm using an SWG so some salt in the pool was not a concern but an aid in getting to the level the SWG required.