You can't generally get the max possible brightness on a multiplexed display. You also have to take thermal considerations into account.
Where brightness is really importand I'd normally not use more than a 4:1 multiplex.
Bear in mind that perceived brightness ( and usuelaly LED output vs. current) is not linear, so, for example, a quarter of the current is typically only going to be approximately half as bright.
White LEDs in particular are very efficient, so unless you want particularly insane brightness it's rarely an issue. Do some tests to see what current you really need.