Here is the fellow who needed high precision with his A3 as he is etching PCBs:
He also was getting distortion due to the belt keeper tension change.
I suspect that it depends on how tight you make the belt. If it is as loose as it can be without slipping, then it will probably be less distorted as it is tightened as it approaches the ends of the run. But perhaps if it is tight in mid run then it will get too tight at end run. Perhaps a smart thing to do would be to set the belt tension with the gantry at end-of-run rather than mid-span as I had done before.
In any case the new belt keepers are designed to position the belt ends at the same approximate height as the bottoms of the top rollers, so that the belt will always run in parallel to the 8020 channel. I have a 3D printer here at work so this is a cheap fix experiment.
Likewise I will be printing out adjustable wheel mounts so that the friction of the trolleys can be adjusted to be zero-play with minimum friction.