GPU boost makes these calculations hard (the stated clocks are not a hard cap), particularly with Maxwell being well known for the advertised speeds being very conservative.
1. Nvidia on their 1080 presentation said 1.7x effective bandwidth over 980 because of the following:
1.4x bandwidth
1.2x compression benefit
Total = 1.7x
1080 = 320GB
980Ti = 336.5GB
980 = 224GB
So 1080 has effectively 380.8GB/s of memory bandwidth. That's only 13% increase in memory bandwidth over 980 Ti.
2. There's only 64 ROPs on the 1080 versus 96 on the 980 Ti.
Let's compare the full specs.
1080: 8.96TFlops, 112GT/s, 380.8GB/s effective memory bandwidth
980 Ti: 6.195TFlops, 105.6GT/s, 336.5GB/s memory bandwidth
44.6% increase in flops, 6% increase in texture fill rate, 13% increase in memory bandwidth. Different games benefit from different things but unless 980 Ti was so imbalanced, the lack of increase on the fillrate and memory bandwidth might be holding it back? 32% increase is actually impressive considering the specs. Of course despite improving marginally on some things the cost is just as much as the previous gen flagship.
Against the 980, it has 80% more flops, 45% more texture fillrate, and 70% more effective memory bandwidth. And it achieves very close to 70% gain.
Based on this I'd call Pascal actually more efficient with equal spec than Maxwell Gen 2.