If have the iGPU active, then the NVidia Optimus driver will display accelerated 750M data on the internal display. It will still output to the HDMI/TB2 port.
Certainly Apple should give the option to allow this switch through. Shelltoe, a eGPU user, managed to enable the GMUX so the iGPU appeared OK in Device Manager but didn't manage to switch throgh the LVDS lines to attach the internal LCD to it.
REF:
MacBook Pro Retina 15" + GTX 560 ti @ TH05
Unfortunately, Apple deactivated the iGPU if the dGPU is present in Boot Camp.
However, if there's an external display attached, the dGPU will be activated, regardless either in OS X or in Windows.
Yeah, Apple should give us the option to have the iGPU driving the internal display as well in Boot Camp.
I'm not sure about this, but perhaps by screwing around with the drivers, perhaps one might be able to activate the iGPU and shut down the dGPU in Boot Camp, hence enabling Optimus to work.
EDIT: On second thoughts, maybe when booting up in Boot Camp, Apple wrote something in the EFI to tell it to disable the iGPU if:
1. Boot Camp is used
2. The dGPU is present
So yeah, it may be a firmware issue after all. Long story short, the EFI has to be hacked into and made to enable the iGPU in boot camp.
Afaik, the way that most laptops implement Optimus (and Enduro) is that the dGPU "runs through" the iGPU, then to the screen. As in, the iGPU is the only one connected to the display in most laptops.
However, Apple implemented it in such a way that both the iGPU and the dGPU are connected to the display. If that is the case, there isn't going to be any way to use just the iGPU in the rMBP with Windows since Windows will assume that if there's Optimus, then the iGPU is where the image data is coming from. Since there's no iGPU (due to the EFI), Windows assumes that it only has a dGPU, uses that, and that's the end of that, since Apple hasn't done anything to accompidate Windows users on their products besides allowing you to dual-boot.