Small amount
Any GPU solution that exists on GPU but is not enabled on drivers, will not work. So imagine you have Nvidia Maxwell architecture card and use it with Kepler drivers. Those Kepler drivers have no support for any new features Maxwell has. Then you switch to drivers that support all new features Maxwell has. Still claim drivers can only improve performance small amount?
Btw. Vega is supposed to have tile based rendering solution, similar to one that come with Nvidia Maxwell.
That (and many other new features that comes with Vega) is not enabled with current drivers.
Polaris came out over year ago.