"Born premature" is an established turn of phrase. You read it, you know exactly what is meant. "Born early" is not an established phrase. You have to think about it more. It could mean "born early in the morning" or "born before her time" or "born prematurely". You can figure it out, it's not rocket science, but it's a little papercut of having to expend some effort. You have to think and pick the most likely meaning.
"Very small when she was born" is readable enough, though you do have to spend a bit of effort discarding the possibility that the author is simply drawing attention to the fact that babies are small to set up some future point.
"She has trouble seeing" is not as clear as the original "...which has impacted [...] eyesight". True, "impacted" is unwieldy and could be improved, but the causal link between the premature birth and the health difficulties is no longer explicit and must now be inferred, taking more mental effort from the reader.