A little insight into the pool store and their usual 'freaking out' about high chlorine levels.
Remember just about all pool stores completely ignore the relationship between CYA and chlorine. If you had 0 CYA then yes, your high chlorine levels could definitely cause damage. *BUT* and it's BIG but...you *do* have CYA and not only that you know what level it is because you tested it yourself.
The CYA is buffering that chlorine and the SLAM level you're using has been figured directly in proportion to your CYA level to be safe for your pool and equipment.
The pool store will tell people to always keep their chlorine between 1 and 3 and at the same time continue to sell them products that raise their CYA through the roof making their chlorine totally ineffective.
It's unfortunate that the pool stores just don't pay attention to CYA but it is what it is and it's why a lot of people end up with green pools and find TFP when looking for a solution.
Good thing we're here to help!