Yes Dynos will tell the full story here. Bmw's ecu's react very differently day to day and also based on how you were last driving it. Even with my old N54 car some days it would feel like a rocket and some days it just doesn't have the same kick to it. Could be due to various reasons.
If the ETG tune is just a piggyback its probably similar to the BMS stage1 or the AFE tunes. They are basically tricking the ECU to push out more boost by giving it false readings through the same/similar signals.
Can comment about the AFE version because i don't know anyone who has it. AFE isn't known for ecu tuning so i would be skeptical honestly. Could be just a rebadged/package of a JB S 1 honestly or very similar.
Current - 2013 N55 535xi Msport SAT
Retired - 2009 N54 335i Msport 6MT
Retired - 2000 M52TU 323i ASA Supercharged 5MT
More Photos of my car here