I just installed the Orella sub upgrade on a Model 3 with premium audio this weekend.
The kit is comprehensive, it includes all required wiring & pigtail connectors, an upgraded 8" subwoofer which installs into the factory enclosure via adapter ring and a dedicated amplifier to drive the new sub.
Installation is straightforward, I'd call it light-medium difficulty for the average person. It's easy for anyone familiar with taking apart car interiors as this is the most significant hurdle.
Results are what you would expect re-using the factory enclosure with a better 8" speaker simply driven with more wattage (the specs of the amplifier are not provided by Orella): significantly better base performance with added punch in the low range but not anything which dramatically outperforms the rest of the factory premium audio system.
It's not cheap @ $750 but the ability to upgrade my audio system without having to splice into any wiring, lose trunk real estate or risk dealing with fault codes made it worthwhile to me. The new sub easily outperforms the factory setup without getting boomy or feeling mismatched to the rest. The system can also be uninstalled to return to factory if desired.