SPI Speed Issues Tiva C/TI-RTOS

I have slightly altered the SPI Loop Back example provided with TI-RTOS, by just adding a larger arrary of data to send. However when I increase the SPI speed in the MHz range it seems to lose timing.

I need to send at 6.4Mhz, but my LA is reporting 6.858Mhz, the strange thing is, that I can bring it down to 6MHz or less in my settings, but it still seems to be sending 6.858MHz. 

Are there any settings I should be aware of, or have a reached some sort of limit. I am developing on the Tiva C LaunchPad (TM4C123GXL).

Here is my code, it is nothing special.

	    SPI_Handle masterSpi;
	    SPI_Transaction masterTransaction;
	    bool transferOK;

	    SPI_Params spiParams;
	    SPI_Params_init(&spiParams);
	    spiParams.bitRate = 6400000;
	    spiParams.transferMode = SPI_MODE_BLOCKING;
	    spiParams.mode = SPI_MASTER;
	    spiParams.dataSize = 8;
	    spiParams.frameFormat = SPI_POL0_PHA1;

	    /* Initialize SPI handle as default master */
	    masterSpi = SPI_open(Board_SPI0, &spiParams);

	int j,l;
	for (j = 0; j < 3; j++)
	{
		testData[l++] = 0b10101010;
		testData[l++] = 0b11111000;
		testData[l++] = 0b11111000;
		testData[l++] = 0b11111000;
		testData[l++] = 0b11111000;
		testData[l++] = 0b11111000;
		testData[l++] = 0b11111000;
		testData[l++] = 0b11111000;
	}


    /* Initialize master SPI transaction structure */
    masterTransaction.count = 24;
    masterTransaction.txBuf = (Ptr)testData;
    masterTransaction.rxBuf = (Ptr)masterRxBuffer;

    /* Initiate SPI transfer */
    transferOK = SPI_transfer(masterSpi, &masterTransaction);

Thanks,

Glenn.