This is correct to a certain point. Once the resolution gets high enough it's starts taking a toll on the VRAM (or in this case the unified memory pool) but if a game was running 1080p at 60 FPS at 4 GB going to 8 GB will not help the frame rate at all on the other hand if a game was running at 1080p with 30 FPS and the VRAM was tapped out at 4GB than adding VRAM definitely helps the framerate.
A good way to see this is with the multi-monitor setups people have or the new 4K displays. Most of the current gen cards run out of VRAM before the processing power is tapped out. Take a look at these benchmarks
http://www.anandtech.com/show/7120/some-quick-gaming-numbers-at-4k-max-settings , you can see that in a lot of these benchmarks the 7950 performs better than a GTX 680 even though GTX 680 is the faster card, the reason that is happening is the 2 GB VRAM is too little to drive those resolutions.
Back to the PS4, I don't think we will have VRAM problems until the GPU starts doing some heavy duty compute processing. Cerny said in his presentation that they specifically made the RAM easy to work with so developers don't have to feel like they are solving a puzzle. On the other hand Sony doesn't want the console to stop evolving within the first few years so they beefed up the GPU so that the developers who want to go the extra mile have an option to push the system further. Cerny specifically mentioned modifications to the GPU was to make the compute performance stronger.
As for going to 8 GB over 4 GB of RAM, Sony should absolutely thank Gearbox for talking some sense into them. With only 4 GB of RAM I think Sony's console would have been tapped out in ~2 years. Some games on the PC are already pushing VRAM usage to 3 GB+. I know BF3 and Skyrim with mods has some really high VRAM usage and those games bring even the strongest systems to their knees.
I have no idea what the Xbox One has planned because they are so secretive about their consoles specifications.