According Notebook Check the GeForce 940MX, 930MX, and 920MX are all based on Maxwell GM108 and will replace the 940M, 930M and 920M.
Besides changes to clock speeds, the main difference between these GPUs is support for GDDR5 memory, alongside support for slower and cheaper DDR3.
Both the 940MX and 930MX come have 384 CUDA cores and a 64-bit memory bus for GDDR5 or DDR3. The 940MX is clocked at around 1176 MHz, while the 930MX has 967 and the GDDR5 has 1006 MHz. The 920MX has 256 CUDA cores and is clocked at 926/965 MHz for the GDDR5/DDR3 models.
The DDR3 variants of the MX series are similar to the existing M SKUs. A GDDR5 model will have a performance gains because of the better memory bandwidth.
So far we have not heard of any new notebooks signed up for the new GPU, but then it is early days yet. They will probably head for entry-level gaming laptops and mid-range workstations.