The battery will be load tested using the right machine and software. Superior to anything that most parts stores will do. They will also check that your vehicle is running the latest calibration file (software that governs Start-Stop system, smart alternator charging algorithm, etc.).
The replacement battery, like ALL current Subaru batteries, is a EFB design. It’s NOT an AGM. There are reports on other boards of Subaru dealers refusing to evaluate charging issues on vehicles that are not using the approved design battery, so proceed at your own risk if that’s your choice.
The SOA821B900 is in the general size class of a Group 24 battery. Autozone does sell what appears to be the identical product. The Exide/Stryten Duralast Platinum 24F-EFB. 750 CCA / 120 RC / 76 Ah.