Mbed LPC1768 + SPI 16bit

I am using 3-wire SPI to communicate with a display. Because data is sent to a display without an answer I want to write without read. Therefore to increase performance I am using the “BurstSPI” class. In short it just writes out the data without reading.
This works fine as long as the SPI format is 8bit. If I want to switch to 16bit, with every value sent a read is required.
Is there a cause for this?
Or, maybe there is a bit or something to set to prevent that? I don’t find it.