The only thing I've read about them was in Dirt Bike on their project 350. They reckon it didn't make any/much diff to the power.
Methinks the F5/F9 is actually strangled at the point of inlet to the carb so no matter how much you port it, it doesn't make much diff, at least to top-end. I had an F81M engine for a while & there's not a lot of distance between the carb mouth & the carb cover on those engines, where the air has to do 180deg. Just putting a spacer behind the carb cover may well un-strangle them somewhat, but I never got to try it.
R'race guys get plenty of power without the cover