Apparently, yes. I read somewhere that some guy thinks they're real because if you watch their videos *cough* their breasts have natural falloffs and motions relative to their own motions - implants tend to stay still.
On the other hand, it's not really possible to gauge unless you're unlucky enough to examine those breasts first-hand. Which leaves it down to a few select in this world as gravure idols don't do porn. Or they were former gravure idols (Reon Kadena, for example). Or they threw away that tag ages ago (Fuko, for example).
Please don't ask me how I know so much...*