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If it's optimized and works well @ f/5.6, it'll work just as good or better at faster and brighter apertures
Yes, but it may not work as well as the stock Ee-A, or the Ee-S, which was designed for fast lenses. Specifically, it may not show shallow depth of field or permit accurate manual focussing other than relying on the AF assist. For sure, each has its own setting in the 5D's custom function, which is consistent with the f/5.6.
So you might prefer to modify an Ee-A or Ee-S.
Several people on the OM mailing list successfully scribed guidelines in their stock screen using a Stanley knife and straight edge. I wasn't one of them, but I have handled screens from both OM and EOS, and they appear to be very similar plastic.
Be very careful. The plastic is very easy to marr. You will need something like satin fabric under your straightedge (steel ruler) and under the screen to prevent damage. Don't touch the surface either. I can tell you the plastic on the Ec-S is soft, because mine has a blemish from a piece of grit.