I need to see if it really does dip in performance.
I currently have a HiRes 2011 MBP and can't take the "lowered" retina resolution because I am a designer who mostly does not use a 2nd monitor.
No, it tests the power of the primary components. That power is the same regardless of the resolution being used.
Think of it this way. It is like a car that is towing something else. The heavier whatever you are towing is, the slower it will move. The engine, however, is still putting out the same power.
Honestly, just get some cheap or free game and set it to show the frame rates. Change the resolution and look how the frame rates fluctuate and that will give you a decent idea how the overall performance changes with resolutions.