Poor USB2 performance on G4 Mac Mini - expected?

Hi,

I'm finding that I can get at most aound 60mbps from the USB2 ports on
my Mac Mini G4. For example:

# dd if=/dev/sd0c of=/dev/null bs=1024k count=100
100+0 records in
100+0 records out
104857600 bytes transferred in 19.731 secs (5314358 bytes/sec)

iperf with a gigabit usb ethernet adapter (rtl8153) tells a similar
story, getting ~45mbps down and 60mbps up.

Is this expected? Any ideas for investigating what's happening here?
sysctl parameters to tune?

Thanks,
James.