1. Is it possible to let the map to dev from the host that the video card worked effectively? Yes, you can. Will it be effective in this Virtualbox? XS.
2. The losses will be in any type of virtualization, but depending on the type of solution they will be different. I can not say for Virtualbox, but the old article on habré in the tests when compared to the native hardware virtualization in kvm and xen losses accounted for, depending on the type of task, a matter of interest.
3. Virtualbox is clearly in a furnace. You need to dig in the side of Linux (kvm+qemu, proxmox, xen) or in the direction of the free esxi... if you want Windows, it is windows hyper-v server + remote fx, but t-shirts in the latest releases cut functionality.
I take pleasure in proxmox, it's a special way drained debian +kvm control via web interface and console. how to set up article here
the performance did not compare, but the games performance on gtx1070не noticed. Good enough for me.