Do I need a COBB Accessport if I use ATLAS?

No. This trips people up because the Accessport has been the default answer for Subaru tuning for so long that it can feel like required infrastructure. It is not - ATLAS replaces it rather than supplementing it. What the Accessport does is read the ECU, write maps to it, and display gauges and logs. ATLAS does the same jobs using your laptop and an OBD adapter instead of a dedicated handheld. So the practical answer is that you need one or the other, and buying both would mean paying roughly $770 for capability you already have. There is a reason someone might legitimately end up with both. If you already own an Accessport, ATLAS gives you deeper access without discarding a device you paid for - and the Accessport still has genuine advantages, particularly in-car gauge display without a laptop on the passenger seat, and flashing in a car park without setting up a computer. Some owners keep the handheld for convenience and use ATLAS when they want to work on the map properly. But if you are starting from nothing and choosing a path, pick one. The comparison comes down to what kind of owner you are: the Accessport sells convenience and a well-developed map ecosystem, while ATLAS sells depth, custom features and a cost of essentially nothing beyond an adapter. One genuine consideration if you are switching or selling. The Accessport marries to a single vehicle, so it must be uninstalled from the car before it can be used on another - and selling the car while still flashed and married effectively sells the device along with it. ATLAS carries no equivalent constraint, which is a small but real advantage if you change cars. What you do need with ATLAS is an OBD adapter and a laptop. That is the whole hardware list, and it costs a fraction of a handheld device.