Well, confirmed.
Wired directly to ICSP header on due I get full speed!! 
I getting from a Sandisk Extreme PRO (95MB/s):
Type any character to start
Free RAM: 62747
Type is FAT32
File size 50MB
Buffer size 32768 bytes
Starting write test. Please wait up to a minute
Write 4235.57 KB/sec
Maximum latency: 30782 usec, Minimum Latency: 7633 usec, Avg Latency: 7732 usec
Starting read test. Please wait up to a minute
Read 4436.36 KB/sec
Maximum latency: 7791 usec, Minimum Latency: 7373 usec, Avg Latency: 7385 usec
Done
The best speed is obtained with 32k buffer.
I'm running slideshow now in 295mSec per picture, outstanding!!! ]
Would it be to hard to implement the paralell mode in arduino? I don't mind to sacrifice the card reader to have it working this way!!!