Even Kepler was already somewhat more bandwidth efficient.
I think though the delta compression is the biggest reason why Maxwell is more bandwidth efficient, I always thought another reason is the fully unified cache nvidia has since Fermi (as rop cache is integrated into L2), but I don't have any proof for that.
Im no so sure about it, taking whatever it is Hawaii or even Thaiti ( GCN 1.0 ), comparing the 780TI vs 290x, increase resolution and bandwith demand, and suddenly the 290x was take the lead, and lately, with some new games, we see the same scenario with the 980. ( where the old 290x close the 980 strangely at high resolution ).
Maxwell benefit of an SM architecture revised who allow a better efficiency of the SP.. ( basically a 1/3 performance increase on the shader side with same number count ). the rest is the high core speed of Maxwell to take in account : ~ 20-25% higher clock speed vs Kepler. And thats a lot, specially with an architecture who scale extremely well with the core speed. ( OC test is a good start for check it ).
Alone the core speed increase and the better efficiency of Shader processor performance drive, something like 40 %.
Ofc the memory compression algorythm is too responsible of it, thats effectively 100% sure that it help a lot Maxwell bandwith efficiency. This said, the technic have his limitation as shown with the 980.
In the case of the 290x or even 7970ghz, the same old problem is here, less than on Cypress, but the SM need to be feeded correctly and then the bandwith follow. In their case, if we compare to Maxwell, i tend to believe the SP efficiency hit his limit way before the memory bandwith.
There will be a way to compare them on this aspect: downclock the TitanX to the same performance level of the old 290x, then up the resolution and bandwith requirement on different games for see who is limited first.
But lets be honest, it will be complicated, even on the choice of games. Most games follow hardware possibilitty increase, specially if they have been developped with and for the actual architectures.
It is funny to see the suddenly increase on the Vram requirement on some studio titles, when suddenly some gpu's are more capable than what they was on this front.