Eryk, you may have fibrous breasts... I do, and while I can get the fat to mash down pretty well, the breast tissue itself is pretty hard and refuses to bind incredibly well.
However, to speak to the general topic, I've been on T for about 14 months, I think, and haven't had top surgery. Most of the reason is lack of funds, though some of it is that I'm terrified of having surgery of any kind. Mine were about a 36B before I went on T, and I don't think they've shrunk in the slightest; however, they have begun to sag quite a lot. The couple of people I've been around without a binder recently say they can't tell I have them, but I think that since I'm still a tad overweight, I can get away with having what look like bitch tits. I don't want them, though. I wish there were a way to get rid of the things without having painful, costly surgery, but there isn't.
So yeah... you could likely get away with just doing T and never having top surgery if you're really small-chested and don't mind having bitch tits. It will break down the breast tissue a bit, but personally, I think mine sag more because of the 3 years of binding than because of T.
SD