I've tried the following
ICH8 in IDE legacy, windows driver- same
ICH8 in IDE legacy, intel driver- same
ICH8 in IDE native, windows driver - same
ICH8 in IDE native, intel driver - same
ICH8 AHCI in IDE legacy, windows driver- same
ICH8 AHCI in IDE legacy, intel driver- same
ICH8 AHCI in IDE native, windows driver - same
ICH8 AHCI in IDE native, intel driver - same
JMB363 AHCI, windows driver - same
JMB363 AHCI, JMicron driver - same
That's a lot of OS installs!
It seems whatever I try, it's not exceeding 100MB/s at all, let alone average...