Ok, now I feel really embarrassed. It turns out that the computer I was testing from was plugged into a 10Mb hub.
I hope the lengthy debug info posted previously will help someone else for comparisons, and I am still curious about the memory usage.
It looks like I have a fully functional raid5 with 4x320Gb drives. I know I'll get no sympathy if it dies on me
, but it looks like it's working fine.