<p>Can the GT2 belts lengthen themselves if they are tentioned too much?</p>
<p>I had them tensioned quite a bit until I saw the <a href="https://youtu.be/zoKmmT0a7jk" rel="nofollow noreferrer">video</a> from &quot;Lost in Tech&quot;. I then decided to reduce the tension, but the dimensional precision was all over the place. So my guess is, that the belts are too long now?</p>

First of all there are two methods to achieve the belt be tensioned.
First method is when both ends of the belt hard attached. In this case if there is a fluctuation in the mechanical system then it will be absorbed by the belt itself. And in this case with big tension it will result in stretching over time with tension disappearing.
The second method is to use spring at one end. The spring will absorb all the fluctuations with little or no effect on the belt.
But I had really bad problems with GT2 PU belts (including steel reinforced), under big tension they degrade suddenly with big change in the geometry at some position. When removed they look twisted. Looks like some reinforcing wires slipped inside the PU body of the belt.
Once switched to rubber GT2 belts (fibreglass reinforced) I never had problems connected to the belts. I can tell that rubber GT2 belts have no noticeable change in the geometry over many years of constant use under high tension with the spring.