My ex had a BA done with submuscular saline implants. At first they appear very unnatural and super high up on the chest, because the muscle has to stretch to let them drop.
I think it took around 3 months for them to drop, and then probably another 3 months before they were done.
I don't know if it was the size she chose or what, but they always felt a little odd if you grabbed them all the way around the boob. Gently poking with a single finger they felt no different from natural breasts, it wasn't until they were squeezed with full hand that I could tell they were implants.
I think it is important to be realistic about the way they feel. They will probably look great, but they are implants and it will be noticeable under certain handling tests. If that is something that bothers you, I would suggest rethinking. I have to say after she went through that, I know that I don't want implants in my body. That said, I have never really been dysphoric about having smaller boobs nor wanted a BA at all. I am sure it's different if you want to have bigger breasts. My ex is very happy with hers, by the way. I am friends with the guy she is seeing, too, and he likes them a lot.