I've got 3 sites on Google fiber now, one of them on a Asus RT-AC68 w/ Tomato 1.28 Shibby, so maybe this applies or maybe not.
CTF / Cut Through Forwarding- enabling this gave me the same speed as the Google box, typically 850-950m up and down, it varies of course.
It's on the "advanced / miscellaneous" page.
I'm not sure, but some of the comments I read when I was researching it implied that if you were using QOS or possibly some other features that CTF would be disabled. I'm not sure if that means that the setting would be ignored, or what. In this case I've got a simple situation (at least in that respect) and have it enabled and it's working fine.
Also, if I understand right it's a hardware feature, and I'm not sure if all hardware supports it. Might be unavailable on the 66, I don't have any experience with one.
Hope that works for you! :)