If the GTX 960 had a 256-bit memory interface, would that give it a major performance boost ?

Solution
No. To be significantly faster they need more shader count. Nvidia decides to use 128bit memory interface with 960 because they have conpression tech that reduce bandwidth usage. People asking for cheaper card; the likes of 960 is the result. Even AMD themselves limiting tonga to 256 bit despite by design tonga is capable to use 384bit memory interface just like tahiti. And their reasoning actually the same as nvidia: cost.
No. To be significantly faster they need more shader count. Nvidia decides to use 128bit memory interface with 960 because they have conpression tech that reduce bandwidth usage. People asking for cheaper card; the likes of 960 is the result. Even AMD themselves limiting tonga to 256 bit despite by design tonga is capable to use 384bit memory interface just like tahiti. And their reasoning actually the same as nvidia: cost.
 
Solution