Thanks for your reply. From /dev/zero to /dev/null gives the type of numbers I would expect from my SSD 
unmatched ~ # dd if=/dev/zero of=/dev/null
^C2258356+0 records in
2258355+0 records out
1156277760 bytes (1.2 GB, 1.1 GiB) copied, 4.06147 s, 285 MB/s
unmatched ~ # dd if=/dev/zero of=/dev/null bs=1024k
^C3710+0 records in
3710+0 records out
3890216960 bytes (3.9 GB, 3.6 GiB) copied, 2.05212 s, 1.9 GB/s
unmatched ~ # Is there some (known) bottleneck in the PCI bus of the unmatched?