The amount of banana you are adding to your recipe seems about right,
although I sometimes vary the amount I use by quite a bit - if you
don't mash the bananas as much, you can use more as you will end up
with chunks of banana scattered thru your bread...yumm.
As far as the lack of good banana flavor, I would suggest using
*really* ripe bananas -- riper than you would usually want to eat out
of hand. I generally make banana bread whenever we pick up too many
bananas from the store and can't eat em all before they get overripe.
btw - you can save extra overripe nanners, peeled, in the freezer