Hi Hal,
Any chance you can bisect the issue? Any way to prove out if each router/firewall can separately pass traffic faster? I could set up iperf if you'd like to try to try against another GF box from each of your two ends. I've experienced something similar when changing my router's bridging configuration caused it to drop NAT offloading.
I don't think this is it, but figured I'd mention about Google fiber - optically it's PON, not Ethernet. I think the most commonly deployed ONUs are only capable of 2.5gbps downstream and 1.25gbps upstream, so if two customers are on the same color and on the same OLT, their combined upstream traffic can't exceed 1.25gbps up. The faster speeds are a different color/wavelength and technology. A PON duty cycle meter would tell you the total downstream traffic utilization at one end, but they're not cheap, and you'd need a splitter to measure while your plugged in. A standard optical power meter won't do it. But again, I doubt it's GF over provisioning.
If you're feeling adventurous, there's a local serial console inside the original white fiber jack, they were running kernel 2.6.32 and a minimal user space. There are some optical metrics available, IIRC under / sys/devices/platform/gpon/info/ but it's undocumented, and as part of becoming an adult, I decided I valued my Internet stability more than my curiosity.
-Richard