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Well, I am not a big fan of TCs, partly because I do a fair bit of action photography for which I need every ounce of AF speed I can get. Furthermore, I seldom if ever hunger for more FL.
Therefore, I can't give you a good answer. Several years ago I did try 200 f/1.8 with 2xTC and was utterly unimpressed with IQ, let alone a sluggish AF response. Never again have I mounted a TC on the lens since.
One also needs to keep in mind that the 200 f/1.8 AF drive speed is unexceptional (i.e., average) to start with, thus placing TCs on the lens moves you into a sluggish domain.
200 f/2 IS has a faster AF response time for sure.
Yes, when it comes to subject-background separation, 200 f/1.8 is obviously excellent. Moreover, the lens is very sharp wide open, therefore one doesn't stop her down for IQ, only for DoF.