I assume you are refering to the Advanced Load Balancing setting on the NICs. The name is misleading as it indeed only receives on one NIC but sends on both. In the HP controller software the name has been changed to Transmit load balancing and fail-over. to have 2GB up and down you have to go with link aggregation. But I have read in the HP docs that that only works with one switch so no switch redundancy. The experts please correct me with an example config for the 2910al switch.
Did you test your rig? What results are you seeing? See my post on SQLIO results and test settings with a P4300 which has 2x8 disks so less Iops as your P4500 (2x12).
I assume you've set ALB, jumbo and flow control as the Networking Best Practices Guide.
Did you enable RSTP on the 2910?
My config as I recall:
spanning-tree config-name MYSAN
spanning-tree config-revision 1
spanning-tree force-version rstp-operation
spanning-tree instance 1 vlan 1
spanning-tree instance 1 priority 2
(different for second switch)