If your E dose is low, waiting to start progesterone 2 years is better because you'll not conteract your E. With a higher E dose, its less critical but you still need breast tissue so it makes any sense at all to use it.
Also, if you've got little breast projection, starting progesterone is useless in general (say A cup an below); progesterone needs breast tissue to work with, it doesn't improve projection, just the lateral size of the breast.
In GG, progesterone kicks in when the menses start, which is several years, two, after breast growth start. So, if we follow this lead, starting late doesn't make any difference.