Do note that I have not, and I will not, make a judgement or declaration of the realness vs fakeness of Rena Momozono's tits. For Julia, I would say 60% confidence there's no implant. But for Rena: I cannot confirm nor deny.
I generally agree with your earlier points, which is that most large breasted JAV actresses are augmented. Probably around 75% of those with say F cups or larger. But with some women, you can tell there are no implants for sure, and Julia is one of them. While it might be possible for them to add implants and make them look like that, I don't think a surgeon would leave that much hang in them.
Any of these women with huge boobs who got implants that look real did it more for firmness than volume. So, for example, Hana Himesaki whose photo I posted earlier, I think she is augmented in spite of no telltales and total softness when kneeded, because they just hold up too well for breasts of that size. Undoubtedly some like her, or say Ena Koume, whose breasts are identical to hers, had large breasts before and got probably 1 cup size augments. They did this not because they wanted to go from say H to I, but because they wanted firmer breasts. If Julia got implants in spite of already having huge breasts, but didn't want to get a lift out of them, then I wonder why she got them at all.
I think there are four categories of large breasts: 1. all natural (julia), 2. augments but it's impossible or very hard to tell (Hana Himiseki, Anri Okita), 3. implants are obvious but not totally plastic (Asahi Mizuno), o r4. totally plastic (Amami Tsubasa.) Those first two categories are equally attractive to me. The third is nice/acceptable. The last category though - breasts like that only look better than small breasts with clothing on. With it off, most IMO probably looked better with their A/B/C's than with those.
Those other girls I posted by the way, Sachiko and Nanami Matsumoto, both are almost certainly natural. Nanami is borderline BBW. Would be odd if she didn't have at least somewhat large breasts to go with all the rest of that meat.