I don't see any errors there. Keep us updated and let us know what the results of moving the firewall connection from switch two to switch one.
I still recommend getting the firmware up to date on the switches. I would break the stack, update each switch and then stack them back together again. Then test the speeds again.
Update: As part of dealing with a quirky firmware update issue, I went into the web interface and did a warm failover to flip-flop the managment units (by switching the Unit Type from "StackMember" to "Management" on one of the two switches) and after applying the changes and reconnecting, I'm now getting full bandwidth throughput across the entire network!
Something was apparently awry in the communication between the two stack members?
At any rate, the stack seems to be working as it should now.