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I have owned the 85 L, 85 1.8, and Sigma 85. I currently shoot the 85 1.8. It is the best combination of AF speed, AF accuracy, low light ability, IQ and size for my needs.
To answer your specific questions:
1. Never had a 1D3 so no comment.
2. Yes, 85L AF is slow. Not a deal breaker for me, but def a negative.
3. 85L & 85 1.8 @ f1.8 or f2 are very similar. The point of the 85L as I see it is to shoot it wide open (which is awesome).
I am not sure what you shoot at 50mm, but consider 50 1.2 + 85 1.8 instead of an 85L. Comes in cheaper, and may be a more versatile option depending on what you shoot.