King Street already has Transit Signal Priority (TSP), as does every other streetcar route. The only exceptions are a handful of major intersections including Queen/Queensway, Spadina, University and Yonge.
Adding more streetcars will make the system even less effective, not more. The main issue is that passenger loading times are variable (making it virtually impossible for the system to accurately estimate when the light needs to be green), but a secondary issue is that there are so many streetcars that they start conflicting with each other. Green lights can't be extended forever, so a streetcar could be denied priority by another streetcar passing through in the other direction.
To manage the extremely-frequent service, the solution is conditional priority. Streetcars which are too close to the one in front would be denied priority, in order to space them out, and to increase the chance that other streetcars actually get priority. And with fewer priority activations, we might be able to get priority at those intersections currently lacking TSP.