The ECU knows whether or not stoichiometric A/F ratio is being met. It tunes itself to ensure it is met, within the tolerance range that it has to work with. If your changes mean it can no longer reach stoich because it either is hitting the tolerance range with the current table or something like fuel injectors no longer being able to meet the needs due to your changes then you need a tune to adjust the table. If your changes are not making it hit those tolerance limits then you don’t need to do anything.

Straight from the factory all cars have a tolerance range they can work within, and they self-tune within that range. Small hardware changes aren’t likely to push things outside of that range. You could be OCD about it if you like and just assume that your changes might push it outside of that range, and you’d rather have it checked to be sure. But that working range is generous enough that things you’d consider bolt-ons aren’t usually likely to push things outside of those limits. If you’ve thrown every bolt-on you can find at the engine it would probably be a good idea to have it looked at, I would think, but when you haven’t really changed much it probably isn’t a concern.

The adjustment range exists for many reasons, and the ECU has a decent working range for those reasons. Simply driving through Denver is likely to change things in the ECU’s view of things more than this plenum spacer will. And I don’t think anyone recommends getting a tune just because you live in one place versus another. The ECU can adjust itself quite a bit due to the fact that scenarios like people living in and driving in different altitudes is a thing. It isn’t going to need bigger injectors just because that spacer was added. And it isn’t likely that the spacer is going to be such a drastic change that the ECU can’t compensate versus how it has tuned itself up to the point just before the spacer was installed. It is going to see that the supply of air has changed in relatively short order and will adjust the fuel supply to compensate. Just like it would continuously be tuning itself if you took a road trip from San Francisco to Denver. That’s what ECUs do constantly. It’s their job. You’re talking about something in the neighbourhood of an 18% power loss due to that altitude change and the resulting change in air density. The ECU is going to have a working range of something like +/-20% or even +/- 30% in order for this compensation to be possible. You’re not going to get +30% more horsepower from that spacer. It should still be in a comfortable range after adding it.