It does not cause considerable slowdowns in comparison to fps you get while getting close to 3.5GB utilization. But if entire memory got same access condition (each bank accessed via override), your games would have run around 30-50fps at best. That is Why I joke, that those who complain so much should e allowed to use entire memory at all times even before card utilizes 3.5GB, so they can randomly load their small 400MB into slow region and wonder why it has 45fps instead of 144fps. But question is about real bandwidth as 256bit bus in 980 should perform same as 256bit bus in 970 running same clocks. Unfortunately 970 has 224bit bus effectively. Which can be seen as false advertising. And if not then expect to see GPUs marketed as 512bit bus where you have 16x32 (512) bits going into memory, but controller itself would be only 256bit having effective speed of 256bit.