8-15% is huge. In most large industries, fortunes have been made (and lost) because of fractions of a percent improvements in efficiencies.
Also, think about all the secondary impacts. 8-15% of industrial scale power all comes out as heat. Large transformers have active cooling so they don't melt. This cooling takes energy to run. The cooling forces spacing in the coils which reduces efficiency.
Actual power over power lines is not always consistent. Therefore, the heat generated by losses is not consistent. Different heating rates will change the temperature of the wire which will cause thermal stresses over time. This leads to more maintenance.
Then think of non-industrial scale power uses. EV charging comes to mind. One of the major limiting factors in DC fast charging is a compromise between the size of the cable and heat generation. A large cable is difficult to manipulate. A small cable is easy to move around but generates a lot of heat. Current DC fast charges use active cooling in the cable to push more power through a smaller wire but this still puts an upper limit on the Amps you can push. A superconductor like this can allow for dramatically higher currents (not just 8-15%) through a similar sized cable.
Other electrical superconductors have also shown the property of thermal superconductivity. Basically, a change in temperature at one point is reflected in the whole mass at the speed of light. This would allow for VERY efficient cooling in a wide range of systems. You have one chiller and drop fine wires into different areas to draw out the heat.
I am reserving judgment on the merits of this specific paper but the benefits of a high temperature super conductor that is relatively cheap to make are hard to UNDER estimate.