The evidence that progesterone helps complete breast growth and maturity is compelling. Combine that with the thousands of trans folks testimonials and you have a pretty solid case.
Speaking for myself, P has been an absolute godsend. I had pretty much given up on any further development. Now they feel entirely different both to touch and grab, and from my end, how the sensations are perceived.
But as always, YMMV, and you own your transition. Nobody else does.