This seems like one of those correlation does not equal causation things.There have been multiple studies showing better conversion rates if your average review is less than a perfect 5.0. But that's about consumer behavior, not how Google's algorithms treat your business.
Surely that depends on (a) the industry and (b) the total number of reviews.I think reviews only with 5 stars look suspicious. The key moment is trying not to get lesser than 4 in average rating, because after that we have significant drop in conversions. This research illustrates it well.