the atmega-hardware can do 400 KHz, but you have to tweak the Wire-library in file hardware/libraries/Wire/utility/twi.h.
Near the top of the file you see :
#define TWI_FREQ 100000L
If you change that to
#define TWI_FREQ 400000L
The I²C bus should run at 400kHzBut
you also have to the delete the files
because the library must be re-compiled before it uses the new speed.
(This is done automaticaly when you open the Arduino-IDE)