Interesting. I had a chance to do some testing and I found that if I connect my 2 nodes direct, I can get an active_mtu of 4096, but if I go through my switch (Voltaire ISR9024D), it drops to active_mtu of 2048.
In addition, the default MTU from ifconfig ibo shows as 65520 and I get better speeds if I leave it be through iperf.