mmmmmm.... I put the v4.7 firmware on my V5a, now I am getting 1MB/sec from my unaccelerated A500 with GVP HD8+ and 8 megs of fast ram. I think this is up from 800kBytes/sec for an older firmware (maybe something like v4.2)
Just FYI, using the built in A3000 scsi (same western digital scsi chip as the GVP HD8+).
I got 2Mbyte/sec from the Mechy reader and 1Mbyte/sec from the SCSI2SD