I just found a neat trick for my rMBP with 750M, which I thought others might enjoy. The new Photos app from 10.10.3 has a unique feature not found in any other program, to my knowledge. When plugged in, the dGPU activates. When unplugged, the dGPU deactivates. So, if you keep the Photos app minimized at all times, your computer will automatically switch between dGPU and iGPU depending on if your are plugged in. Honestly, this is exactly how I want it to switch in the first place. Best performance when plugged in and battery-saving with unplugged.