Yes, the one line will only change DLYBCT to 0 and keep all other settings. I'm still quite green at "C" and digging under the hood of Arduino. Just starting to use the arrow notation in spi.c. To me, its descriptive enough, but knowing where all the variables are set and a list of all possible arrow notation commands would be good to have. I find ATMEL's ASF and website good resources also.
I've also seen the DMA SPI on Github. I think DMA would be beneficial for your application, especially as you get in to the high end of SPI clock rates. Unfortunately, I've never used it and am waiting for more examples.
Good luck with your project ... and by the way, All I can guarantee is that the code "should" copy and paste OK - that's it. (after seeing that the LM96570 chip is used medical ultrasound applications) 