While there is no bandwidth limitation in full duplex, it is unlikely that traffic exactly fills both directions at once. One is usually constrained in one direction before the other.
You mentioned gig ethernet, where the internals of the computer often constrain communications far before the limits of the network. 200 meg to 400 meg is common on low cost PC and Macs. Hard drive speeds, PCI bus speeds, and CPU utilizaion from cheap ethernet NICs are common culprits.
I tried to remain child-like, all I acheived was childish.