I am currently running a 40 gallon breeder mixed reef with a 20 gallon sump. Is it worth it to set up an AWC using the Dos for a small system like mine?
If the DOS gives you regular water-changes and without it you don't get regular water-changes, then "yes". If you will perform manual, regular water-changes (for years and years to come), then "no".
I chose to implement a DOS-based AWC on my new tank because my previous, first tank declined over the years from me not performing regular water-changes.