
And I don't agree with what you said "The extra stop you get by shooting at f1.2 you effectively doubles your subject isolation you'd get at 1.8 (giving you more pop!)".
It depends on the scenario. If the focal distance is short and the background is far from the subject, which is quite common in many portrait shots for example a close-up shot at close to minimum focus distance, then f/1.8 aperture will give you the same look just as f/1.2 simply because the background will be completely destroyed.
It’s actually pretty rare that I’ll shoot a portrait with the 85L close to the minimum focus distance, and if I do, it’s usually stopped down to 2.8 or more because a) you don’t have enough DOF to keep all of your subject in focus, b)the background will be compressed beyond recognition.
That said, I primarily use the 85L wide open for loosely framed shots (full and half body portraits). For this type of framing, the background is very much within the frame, and f1.2 creates a sense of dimension (pop) that a smaller aperture flat-out can’t.
As mentioned earlier, the jump from f1.8 to f1.2 is quite a lot in terms of subject isolation. It’s a full stop. Its equivalent to the change you get going from f4.0 to f2.8, and close to equivalent to the change you get going from f2.8 to 1.8.