I have pretty bad eyesight, so I rely on AF at least 85% of the time. Is it really soft wide open on portraits? Just curious what you meant by using it at 2.8 I "will probably not be very happy"
The depth of field will be very shallow at f/2.8, and if you don't absolutely nail the focus to where you want it to be the photo won't be satisfactory. Since the best way of doing this is manual focus, small rotations of the focus ring at portrait distances will have a large result in where on your subject the focus falls. So, manual focusing for portraits will be a hassle. That's why I say you may not be happy. But maybe you are not as ham-handed as I am, and you don't mind just barely touching the focus ring to get the focus to get what you want.
My handy little DoF calculator says that at 1m distance the DoF is 1.5cm when using f/2.8. At f/4 it increases to 2.1cm. Remember, f/2.8 is just one f-stop open from f/4 - not all that much, really. As for improved bokeh, maybe you should look at lens reviews for that - I don't really care all that much about it, myself.
If you are good at placing and using your AF points then this might not be an issue for you.
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Shane