What is the scientific opinion on plus lens therapy?
Plus lens therapy can indeed reverse myopia, but it does take time. Your eyes did not develop myopia in a few days.

The eye represents its environment. Fitting chicks, monkeys etc. with minus lenses always INCREASED the amount of myopia, while plus lenses always reduced myopia and resulted in hyperopia (lens compensation).

Putting monkeys in a confined environment showed a myopic shift. Monkeys in an open 
environment showed a refractive state about +1.00 diopters.

IMPORTANT: As I have said: Unfortunately, it does take time! Do not expect results after 1 week of plus lens therapy. That is why so many people give up and think that myopia is irreversible. You also do not build muscles after 1 week.

Kind regards.

EDIT:

Optical defocus and effects in HUMANS: http://www.ncbi.nlm.nih.gov/pubmed/20592235
Defocus on chicks: http://www.iovs.org/content/44/7/2818.full
Monkeys in a confined environment: http://www.iovs.org/content/2/6/571.short