The difference between f/5.6 on some of the older consumer lenses and f/8 on some of the newer consumer lenses is that older ones really only opened to 5.6 for focus. They had to step down for good results. The RF 100-400 shoots wide open at f/8 no problem.This is why when I eventually buy an RF body I'll be sticking to my wider aperture EF/EF-S lenses, some people also forget that you get a slightly more shallow depth of field as well with the wider aperture EF/EF-S lenses. I understand the reason for some of the f/6.3 EF-M lenses to keep it portable and light weight, but I think the entry level RF lenses should have been the same or better then all of the EF/EF-S equivalents.
I personally see the RF-S 18-45mm f/4.5-6.3 IS STM as a bit of a missed opportunity to get the EF-M 15-45mm f/3.5-6.3 IS STM users to upgrade. In my opinion I think they should have made the RF-S at least the same specs or better. But they could always release one with the same specs sometime down the track.