I know what you mean, I always got 4 hours easily from my bumgenius with nothing apart from its insert but they've started to leak on me recently. I think it just happens as they get bigger!
IMO the only answer is more boosting unfortunately
At least if you use something slim like hemp it won't add too much bulk.
I plan to buy lots of hemp from babeco and make boosters (and the g diapers inserts Karen came up with!)
Bamboozles still last four hours with Matthew atm with no boosting but I expect that will change in time and I'll have to boost them!..
Let us know what you do and how you get on.