I never tried spec driven development for myself, but if I review other's MRs I am typically exhausted after the first 10 lines.
And there are hundreds of lines, nearly always with major inaccuracies.
For myself I always found the plan mode to work well. Once the implementation is done, the code is the source of truth. If it works, it works.
When I want to add more functionality or change it, I just tell the agent what I want changed.
I doubt walls of semi-accurate existing specs are going to be beneficial there, but maybe my work differs from yours.