The most powerful of Nvidia’s new GPUs—the GeForce GTX 880M and the GeForce GTX 870M—are based on Nvidia’s 28nm Kepler microarchitecture. Kepler also forms the basis of Nvidia’s most powerful desktop GPUs, including the GeForce GTX Titan Black. The GeForce GTX 850M is based on Nvidia’s newer 28nm microarchitecture, codenamed Maxwell.
Nvidia announces its all-new GeForce 800M line of notebook GPUs | PCWorld
Looks like mobile maxwell is here. Performance increase looks promising. GT 850m will probably be equivalent of GT 765m if not GTX 770m.