Agreed we are going through the exact same problem but we already have roundrobin on every vm host. We are also utilizing 10gb links and with a Windows box directly attached via iSCSI Initiator we see upwards 400-600MB/sec (through the virtual machine no less) but we are struggling to see anything over 100MB/sec on the actual VMDK disk.
We tried to tweak the iops values, traffic shaping, etc. but we could achieve no more bandwidth.
We also are using 2 switches (Force10 s4810) with 4 active paths on each host