gigabit ethernet speed on 3550

I am testing gigabit speed on a catalyst 3550 with 2 hosts using the g5483 gbic convertor. i am only able to get max 600mbit and average

the Catalyst 3550 is 24port 100mbit with 2port gigabit and standard IOS image

-- out of the box config with ip on the vlan 1 and same ip on two hosts with gigabit ethernet.

-- switchport host on gig0/1 and gig0/2 interfaces

-- no errors/crc

-- i am able to measure 800mbit+ using a dummy dlink gigabit switch

-- i am testing using QCHECK and iperf

thanks for any input.