It will. Until it hits a hard wall due to framebuffer size, then it won't.
1. Assuming its dynamic vram implementation (2 engineers...) is no longer actively optimized for new games.
2. That 4GB isn't actually enough. At 1080 and 1440p? Unlikely anytime soon. There's already games with 4K textures that don't have issues with 4GB GPUs at those resolutions. What's next, 8K textures? You can't tell the difference at those monitor resolutions hehe.
Another thing to notice, Fiji does need AMD to release optimized drivers for game performance to improve over the 390X. Seen it in every AAA title this year so far. Hawaii performs like a champion even without game ready drivers, but Fiji is abysmal without it.