Unfortunately mobility GPUs as used in laptops are considerably slower, typically only ~50% performance, when compared to their similarly numbered desktop counterpart. So comparing an MBP to a desktop PC will always be unflattering due to the relatively puny laptop GPU. As discussed in other threads the Mac OpenGL rendering performance should be broadly equal to the Windows OpenGL performance on the same hardware. Obviously there are greater differences if compared to OpenGL 4.3 or Direct3D on Windows due to the greater rendering features offered by those APIs. Running UE4Editor on a 2012 15" with Nvidia 650M under Windows isn’t an especially fast experience either as that is one of the machines I’m using for testing.
The with a D300 should run the Editor quite well, certainly sufficiently for mobile development. I’m also using a with the D500’s here and it runs ElementalDemo quite reasonably, with very close OpenGL 3 performance under OS X & Windows. I get close to an average frame rate while running with -game of ~25fps, the peaks are ~40 fps and lows of ~15 fps. For comparison’s sake this is somewhere between to 66-75% of an Nvidia GTX 670. The D300 will be somewhat slower whereas the D700 will be faster still.
This is vastly superior to the performance of an Nvidia 650M/750M as used in the MBPs. I’d imagine it is also better than the BTO Nvidia 780M available for 27" iMacs, though I’d love to hear from anyone who’s used that machine as I don’t have one to test here.
It is also worth mentioning that there is still an OpenGL driver workaround in place in 4.3 that can negatively impact performance. This will be removed in the future and should help speed the Mac version up by a few percent.