About 7% additional maximum memory bandwidth - so if you were in a 100% memory bandwidth constrained situation that would be the maximum performance uplift. But being 100% memory bandwidth constrained is extremely rare so it’s probably a marginal improvement. But hey, any extra performance for free is a good thing.
Unfortunately we need to be careful what we wish for. As AI processing becomes more and more popular, if a card like the RTX 4070 had 20 GB vram they would probably all be snatched up by companies to use for AI processing on the cheap, and suddenly 4070 and higher all go for $2000 like the mining craze all over again.