Everything depends on the person, of course, but the "before and after" thread has some truly amazing transformations.
For me personally, I couldn't pass as a guy after a few months on HRT no matter how hard I tried, and I too think the B cup thing is a myth - or possibly a misunderstanding. Cup size varies by band size, so a 40B has the same cup volume as a 38C and 36D, for example. That means that the bigger the rib cage = the smaller the letter of the cup will be, but the *size* is the same. If the average trans woman has a rib cage only an inch or two bigger than the average cis woman, her average bra size will be one cup size smaller.
(Oh, and I'm professionally measured at 38DD, which is another way of saying 38E and the same cup volume as 36DDD/F.)