There is a lot of "secret sauce" about Maxwell which Anandtech wrote about in the GTX 750 Ti review that Nvidia havent shared yet.
The fact that a GTX 750 Ti with 86.4GB/s bandwidth does just as good in 900p as it does in 1600p against GTX 650 Ti Boost with 144.2GB/s bandwidth, means there are some really good optimizations done on the bandwidth part.
http://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_750_Ti/25.html
So I wouldnt worry too much about that the GM204 is only 256bit.
I agree. This can easily be seen in mobile GM107 where a 850m at the same clockspeed with DDR3 gets 75-85% of the performance of the GDDR5 model.
Comparing the 750Ti and the 780Ti on bench, the 780TI is just under 3x as fast. GM204 will have about 2.6x the bandwidth of the 750Ti so it should be able to match.
Comparing to the 770, the 750Ti is just over 50% the speed despite having only 39% the bandwidth.
