Charlotte is 6 weeks old now, and appears to be a fairly heavy wetter (during the day anyhow!), but I have had her in bum genius since she was 1 week old, i use the newborn insert and a bamboo booster during the day, and at night i use just the thick bum genius insert.
I have had no leaks what so ever - and Charlotte pretty much sleeps through the night, I wake her to change her nappy at midnight, and she doesn't then wake til 7am - and they manage that no problem.
I'm sure that as she gets older i'll need to use bamboo or hemp boosters at night as well, but for now - there are definitely no problems with just the thick BG insert.
Hope that helps,