On the face of it, there would seem to be no real advantage in having a faster macro lens other than for non-macro use. The depth of field at 1:1 is so tiny that usually you are wanting to risk diffraction with your f-stop instead of wishing you could open up to f/2. Maybe if you are chasing around small animals in the woods using lower magnification, the extra stop and eye auto focus could be handy. If you plan to use your 100mm macro for portraits, then f/2 could blur your backgrounds a little better than f/2.8. When I first got a full frame camera, I tried that with my non-L version. I and some others here have noted this may be less than satisfactory. I used the term “too clinical” for the look. But otherwise it is a great general purpose short telephoto. Using it as a macro lens, I get great results near 2:1 with extension tubes.
So barring some reliable comparison tests showing the RF blowing away the EF L as a macro, you’d need to have some use case, as I suggested above, where the RF would have some advantage for the upgrade to be worth some money.
Indeed, I generally do NOT need a faster than f/2.8 aperture at 1:1 magnification.
My
usual use of the macro lens at 1:1 magnification is b/w f/5.6 and f/16. Though some times (e.g. to highlight certain small details in flowers, e.g.
parts of stamen, or a specific part of an insect, e.g. the tip of a foot, antenna, etc) - I do shoot wide open and creatively use the very shallow depth of field. There are a number of times I would want and use closer than 1:1 functionality in a macro lens.
However, I do regularly use my 100mm L at f/2.8 at non-macro distances. I will also add here, that the L is vastly superior to the non-L in terms of AF accuracy at non macro distances (and I have confirmed this with multiple copies of both the nonL and L 100mm macro lenses). Also, the rendering of the L is slightly better than the non-L, but both lenses are very similar in terms of absolute sharpness.
I have used the 85mm f/1.8mm USM, and it's a very good lens (particularly for the 'budget price'). And yes, it covers that useful 'short-ish' portrait length, and it does have slightly more potential for portrait subject isolation (blur). But I find with a creative mind, one can use the 100mm f/2.8 very effectively (and I'm far from the only one). I have also used the 100mm f/2 USM, which is very similar feel to the 85mm f/1.8 in practical use.
However I find using my 50mm f/1.8 STM and my 100mm f/2.8 L macro are a great two-lens / two-focal-length combination, giving a lot of flexibility for event photography, along with a standard (or wider) zoom in the bag. for events. (I have shot many many events with this 3 body/lens combination). Hence why a RF 100mm f/2 2:1 would help me cover even more ground!
But as I wrote above, I expect to be using my EF 100mm f/2.8 L IS USM lens on a R5 for a long time to come!
Regards
PJ