We have driven our 2018 Si6 model now a little over 1700 miles and both my wife and I have noticed this week a significant increase in engine power and sound. I was wondering if in modern cars, the break-in period is controlled electronically via engine mapping and if the "engine release" is a one- or multi-step process. Also, how does this impact gas mileage?

 

No
Most engines gain some power as they break in.
A more agressive break in results in more hp as well.
Gas mileage may or may not improve.

 

I thought so too. I last drove the Disco on Sunday (4 days ago - it's our family hauler) and everything was as usual. Yesterday, I drove it again and after letting the engine get up to temperature, I immediately noticed the much louder engine sound while pushing the pedal to the metal, a better throttle response and the much higher acceleration - a striking difference to a few days ago. After coming home, I told my wife and she was "YEEEEES, I noticed too, but forgot to tell you". Does this sudden change in engine performance not speak against a gradual power increase during break-in?

 

It all depends upon the individual engine.
It's overall design to start with, then it's own tolerances/clearance.
Take 2 identical engines, and one will always make just a little more hp than the other.
Sometimes it's noticeable, sometimes it isn't.
The lazy break-in procedures most manufacturers recommend are to keep the average idiot from holding a 2 mile engine in the red line. But there are better and more agressive methods to make rings seat better, and other parts wear to fit better, resulting in more power, and usually better mpg, even if it's only a very small amount.
An oil change in the first 500 miles or so is always a good idea too. It gets all the fine metal particals out, that come from new parts wearing into each other.