So, those claiming this is a hardware fault, here is some food for thought.
The GTX970M is 80TMU and the GTX980M is 96TMU. Maths, a 3GB GTX970M should have the bandwidth falling off after 2560MB. A 4GB GTX980M should have the bandwidth fall for the entire last GB.
GTX980 = 128TMU = 4096
GTX970 = 104TMU = 4096/128*104 = 3328
GTX980M = 96TMU = 3072
GTX970M = 80TMU = 2560
Can someone run nai benchmarks on laptop versions? If the bandwidth doesn't fall off then it might...