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The logic of "I would lose 2 stops, that would defeat the purpose of getting an expensive fast long lens" does not make sense. If you want a polarizer, you are going to loose light. If you put it on a slow lens, you will be that much darker than putting it on a fast lens. Having the fast long lens allows for easier use of a polarizer in low light, and gives you the choice of what you want to try. Changing out a polarizer on a super telephoto is actually faster than changing a screw on polarizer. And remember that the newer camera bodies have several stops more light sensitivity for the same quality of image, so putting the ISO up a ways can be mixed in as an option.
I love my Canon drop in polarizer on my Canon 300 f2.8, and use it in a lot of situations, including wildlife and landscape, as well as most any shot that may have haze from having distant backgrounds. At times I also even use it with a 1.4 extender. I have never bothered trying to upgrade the glass in the drop in polarizer to get less light loss, and may consider that some day, but for now, it all works great for me.