The pro lenses will always be faster than mid range tele lenses. The 100-300 mkII won't be quite as snappy nor as bright as the Oly 40-150 pro, but as you say you plan to use that more so for daylight shooting so should be just fine. A lot of people who switched from FF to the smaller, lighter M43 gear are very surprised by the performance and quality. I really believe that many [we see it on here all the time, you only have to peep into the latest FF mirrorless threads] just buy the latest higher end gear because some reviewers tell them to, or just to keep up with the Jones's - as your hubby discovered, it's more about the grade of gear within any system that counts more, and also the user skill. With Full frame set ups it's no different, for shooting sports you need the faster, higher end lenses where cheaper zoom don't cut it. The real 'big' difference though is the equivilant for FF weighs a tonne more and will be much more expensive.
He has still to try the M4/3 set up during a real match but after last night he seems far more confident in doing this ( except for this weekend as it’s a Cup Game!!). I can’t win!