Standard mirror for MF at f1.4 ?

Is the standard focusing screen of the 5D okay for manual focusing with fast lenses ?
I plan to buy a Rokinon 85 1.4 but I don't really want to change the focusing screen. I'd rather save up and buy the Sigma 85 instead but I need something fast for now.
Btw I don't know how much does AF help at f1.4. Focus recompose should lead to focus plane shifts and the 5D's AF points are all cramped in the centre.
(my old 1D was great regarding focus point count/placement)

I think you mean the focusing screen, not the mirror. They arent very expensive and easy enough to put in as well. People seem to like the screen replacements, I haven't really used any of the MF screens myself though.

The standard issue focusing screen will reveal correct focus to about the depth of field present when shooting at f/4 or so. You can't get it much more accurate than that.
Still, the human eye and brain are pretty good at interpreting the change in sharpness when you rock the focusing ring back and forth around the correct setting, then settle in the middle and thus kind of interpolate the correct focus setting.

On a camera supporting live view, you can focus very precisely by magnifying the screen and then look at it.

Focus shift due to re-composing aren't too much to worry about with an 85 mm lens. It's worse with wide angle lenses, since you can sway the camera much more and still have the subject inside the frame.

Focus screens like this might make a difference.

http://www.tmart.com …Ojeu4qgyLkCFQ_N​OgodJA8Afw

They will have disadvantages though. You can get Canon-made screens too that are for f2.8 lenses.

If you upgrade your camera to the MKII or MKIII in the future, you gain live view, and you can digitally zoom 5x and 10x which is outstanding for manual focusing.