Hi.
1- As far as I know, full duplex mean double the speed, but over 2 directions! That mean 2GB/s in one direction and 2GB/s in the other, NOT 4GB/s in 1 direction!
2- By using 2 GPU, you don't loose the x4 PCIe slot, you only loose the 1x. The x4 is 2 slot away of the first x16 slot. The second x16 only have a regular PCI slot after it.
3- The second x16 slot from the CPU is actually the first one to use in a single GPU configuration. You MUST put the little black board in place in the first slot in order to get x16 lanes on the second slot. If you don't, the second slot will work at only x8. If you put your single GPU in the first slot, it will work at only x8. You mention not noticing any drop in speed. Confirm with GPUZ that you GPU is running at x16.
4- You said "This is not something that can be done with H55/P55 or H57 motherboards as an additional PCI Express controller must be installed."
There is no such thing as "Additional PCI express controller"! What you have are PCIe Switch / bridges, such as the PLX or the NF200. Those devices MUST be linked to the system through EXISTING PCIe lanes!
The Gygabyte implementation of USB3 /SATA3 on the P55 platform is to re-allocate x8 lanes of the x16 lanes from the CPU to this this task. That disable the second x16 slot and reduce the first one to x8. Asus use a PLX switch to bridge 4 PCIe 1.1 lanes into 2 PCIe 2.0 lane from the P55 chipset. They preserve the full x16 lanes to the CPU, but at the cost of a huge bottleneck between the PLX and the P55.
Regards,
Ramon