OK - got the DMA code done.
Writing and reading 32768 block (32k)
Took 18764us to write and 17508us to read
By my calculations that would mean transferring 16 x 32k blocks which would be 1/2 Megabytes would take 300,224Us (1/3rd of a second). (plus a little bit of overhead!).
Transferring 1byte at a time took about 13.7 seconds!
According to @peekay123 (See Post by Paul) Based on my results, is not a lot of speed difference between FRAM and SD Card! I believe I am pushing data out at full SPI speed - and I would suspect that mode 0 on sdfat is also pushing data at that rate - so the actual limiting factor is getting the data to the device!
It has been an interesting experiment however. I will tidy up my library and make it available.