even the biggest of cams on a standard ecu lean out to a decent afr of about 12.5 at top end, the mid range afrs can be in the 10-11 range, which is where most of the tuning gains will be made. most of the top end gain will be through timing and not fuel.

6 cylinders are a totally different beast to a V8, especially with their BBM intake which seems to cause midrange richness in modified cars

The BBM richness is in the factory tune - there is also a big dip in spark around the BBM switch point due to the lean spike from the additional air when the flapper opens. Both can be addressed with a tune, whether using the factory ECU or aftermarket, but if you add too much timing at the switch point you can get pinging (like when running a BBM on an EA-ED ECU).

As others have said, the main gains to be had are in the midrange, making the car much more lively to drive. There will be some peak power to be had but not a lot as most of the time the AFRs are in the power zone anyway.