In reality it was a 400mph bird, side by side with a F4u1 there was nothing to differentiate (much as each company WANTED theirs to be better). Their max speeds were almost the same. In here it seems we may have an F6F that was tested with an inaccurate pitot placement. There were some errors in some of the tests. I'll leave that up to the Hellcat experts on this forum to go over (I'm only remembering what they have already discussed), but essentially it's a 390mph plane in Aces high, instead of 400+ like its corsair brother.