It will improve performance. It will not solve the 3.5GB problem.
If you are playing games that require more than 3.5GB of VRAM, you will still see a slowdown, but it would be faster than a normal 970 when using more than 3.5GB of VRAM.
Will Overclocking (by increasing the frequency of the videocard's memory) improve my GTX 970?
It will solve the 3.5GB problem?
The 3.5GB Problem can't be solved by overclocking or drivers. But it only appears, if you play a game which uses MORE than 3.5GB VRAM, often are this games with many mods