More suggestions from John and my reply.

> we also don't use the VME back plane - it's just too slow - mixing VME commands to plain modules and digitizer modules is unreliable....

> We use CAEN fiberoptic version 2 to talk to the digitizers directly, we have upto 12 digitizers, and can use all channels for several hours, and can fill to about 75% 
of the A3818 bandwidth... 

So far we are limitted to 30 MB/s, if tested with CAEN examples, for example, the wavedump program by CAEN.
I think is kind of the limit by IDE hard drive.
Unfortunately we are still far from that limit, only ~ 1 MB/s now.  :(