I've noticed in the Guide that, while the Repair Facility can receive upgrades that increase its ability to repair damaged units, Mobile Repair Turrets in general seem to lack this feature altogether, meaning that, as matches drag on, their ability to repair damaged units decreases as they grow in HP (thanks to researching new bodies and Composite Alloys upgrades), and as enemy firepower increases, which, in turn, would translate to an ever-increasing reduction in their usefulness.
Sadly, I believe that, currently, the only way for modders to be able to rectify this problem is for them to provide more advanced versions of Mobile Repair Turrets that provide ever-increasing repair speeds (similar to how there are more advanced versions of Command Turrets available, each with more HP and better range than the previous one). No support for upgrading Mobile Repair Turrets seems to exist as of now.
![Sad :(](./images/smilies/icon_sad.gif)