Hi, this is hard to test because you need about 6 Million faces to get over 3.5 GB and this need CPU time before the render even start.
Another point is every system use VRAM for display so a test file work on one system but not for a user with two displays, for example.
The slower part of the 970 is still 10 times faster than the PCIe bus and it take longer to get the data back to the CPU and display it than move it 5 times between VRAM and GPU core.
I think it does not affect rendertime at all.
May a user with better tech background can shade i Light on it.