Stretching springs only increases tension temporarily, only a couple loads/unloads. The way stretching affects the metal, it actually serves to weaken the spring. The better fix would be to shim a weakened spring (probably under the floor plate in this application).
In other xtra power wolff springs, they usually add an extra coil or two, ensuring minimal added thickness, but more power... Keeping both within spec to minimize any potential for cascading issues...