Big drop from the ECU to the alternator.
Stock loom?
What are your injectors seeing in voltage?
This could affect your performance.
What i mean by this is, your ECU uses voltage compensaion to control your injector opening times.
If you voltage drops, your injector coils would be getting less power therefor reducing the amount the injectors are open. Your ecu is clever enough to recognise this.
So, if your injectors are seeing 13.7v when on load but your ecu sees 12.2v, your ecu may be compensating for something not required.
I'd try and look at how you could get your ECU to see the same voltage as your alternator output. Most likely related to a bad supply to the ECU or a bad ground connection fom the ECU.
Our cars are getting old. My AMG had the same problem. I fixed it by adding a few additional earth connections from the block to the chassis and cleaning the areas where the existing earth points were. Plus torquing the earth lugs with star washers that peirced the steel when tightened up.
I also installed one of those voltage stabilisers (capacitor) which actullay made a difference!
Good luck!