The impact was amazing:
Overall cpu load dropped from 2.x to about 0.8. Key to this was the reduction of I/O wait (writing to SD card) from 25% to below 5% in cpu utilization.
So far the combination of Banana Pi and SATA SSD can be highly recommended.
Be aware of power issues if you plan to use a normal (mechanical) hard disk. The bPi is only able to supply the 5V part of SATA power. Mechanical hard disks usually utilize 12V for the mechanical parts (disk spin motor, head control) and 5V for the electronical part. SATA SSD's do not have any mechanical parts and in general they do not require the 12V supply power to be present. For running an bPi with SATA SSD the bPi-specific SATA/Power cable is sufficient in most cases.
cheers - Robert