We broke the story ... the power couple just scooped up a $40 million waterfront estate on Miami's super-exclusive island 'hood, and our sources there say the locals might be more excited about it than J Lo and A-Rod.

We're told current Star Island residents believe J-Rod's arrival will bring back the lure, luster and celeb status the area's been lacking lately ... and, therefore, send property values soaring to new heights.

It's interesting because some major A-listers still call Star Island home -- Diddy and Gloria Estefan come to mind -- but others have sold and moved away in recent years ... like Shaq and Rosie O'Donnell.

So, our sources say J Lo and A-Rod more than fill that void while bringing even more glamour than ever ... along with a nice wholesome ambiance with their blended family.

We're told the other stars on the Island will also be thrilled if the soon-to-be-wed couple ended up paying the asking price of $40 mil ... because, once again, it's great for everyone's bottom line.

There are still 2 properties for sale on the island, and rumor has it ... Jeff Bezos is eyeing both of them.