As there are very few GPUs with 8GB onboard and each game is different, it's difficult to make any conclusions. The only half-way decent testing I can find is this
review of the Vapor-X R9 290X 8GB published a few days ago. Yes, it's Tom's, so the usual caveats apply.
It doesn't look like 8GB will help you much in 1920x1080 with recent games. It really does depend on the game, how its designed. For example, I've read that Watch_Dogs actually alters visual details depending on the resolution you play at.
If you're playing at 3840x2160 or multi-monitor resolutions, then more VRAM is better. But, you'll have to turn down settings to get playable framerates with any GPU available today. It's questionable whether more, lower-quality pixels provides a nicer experience than fewer, higher-quality pixels. This is down to personal choice, of course.
If you're playing at 1920x1080 or so, an 8GB 970 seems a bit pointless to me. If you want more performance, you should either go for a 980, or wait a few months for R9 390x and GM200 products.
This fixation some have with finding the "one GPU to rule them all" is very weird, in a tech area that moves so fast and has such a healthy second-hand market.