Maybe you mis-read the results. The 50MB/s quoted looks like the read speed from FAST ram to me...the video card is doing:
$40000000 readw 690.1ns 2.9MByte/s
$40000000 readl 690.1ns 5.8MByte/s
$40000000 readm 693.7ns 5.8MByte/s
$40000000 writew 538.2ns 3.7MByte/s
$40000000 writel 538.7ns 7.4MByte/s
$40000000 writem 538.2ns 7.4MByte/s
Assuming $40000000 is zorro video card RAM....this would be about right for ZIII I think.
Steve