The guide you read was completely accurate until about 10 commits ago. It hasn't been amended yet to stipulate that from a performance perspective D3D is probably the best backend for Nvidia cards right now. There were a few notable bugs in D3D rendering, but many have been fixed by the very commit that caused the slowdown. So, absent a particular bug or situation, I'd suggest D3D for an Nvidia setup unless you have a Titan card (you don't).
You might not see 50% everywhere, but in motion areas or areas with lots of textures, you will. IMHO that makes the game unplayable.
I couldn't say at this point whether your CPU or GPU is constraining your gameplay. To find out, reduce your IR and see if framerate goes up. If it does, it is a GPU bottleneck (possibly still tev_fixes related, D3D still takes about a 5% hit). If it doesn't, it's a CPU constraint. Although many people will recommend disabling TurboBoost on laptops to prevent overheating, you might want to turn it on and check your dynamic clock rate with a turbo boost monitor. That extra 400 MHz comes in handy.
You might not see 50% everywhere, but in motion areas or areas with lots of textures, you will. IMHO that makes the game unplayable.
I couldn't say at this point whether your CPU or GPU is constraining your gameplay. To find out, reduce your IR and see if framerate goes up. If it does, it is a GPU bottleneck (possibly still tev_fixes related, D3D still takes about a 5% hit). If it doesn't, it's a CPU constraint. Although many people will recommend disabling TurboBoost on laptops to prevent overheating, you might want to turn it on and check your dynamic clock rate with a turbo boost monitor. That extra 400 MHz comes in handy.