There is power to be made with a tune. Why? Because oem tunes are designed for emissions and durability. So minor changes to fuel ratio and spark timing can yield minor gains at the potential expensive of these attributes. Usually at higher rpm. Also because there is some variance engine to engine and a custom tune can dial in this variance. So usually minor but in combo with intake and exhaust can add up some because now you're starting to add more air, and this is really the key. Note these latter two are custom, not off the self tuner in a box.
When this really takes off is when you can significantly modify the displacement of the engine (amount of air the engine can ingest). Cam, heads, stroker, forced induction. A stage 1 tune on my '99 turbo I-4 Audi A4 went from 155hp to >200hp, with consistent gains across the rpm range. This was due to increasing boost and then matching fuel and spark to the increased effective displacement. I never dyno'd my stroker 2FE but I did tune with a programmable AFM to match fueling to the ~10% displacement bump and increased static timing using an aftermarket knock sensor to control detonation. A stage 2 on my VW R would blow the doors off.
But here we're not talking about these kinds of mods. So best case is very minor increases at the risk of undoing the hundreds of hours of calibration the factory did. Pass.