Aren't all the other F4U's faster than the 1? If so then a slightly faster rate of closure might be enough to be noticable. Muzzle velocity is muzzle velocity after all and added to the closure differential you should hit a little bit harder. At the extreme connecting on a HO shot should hit a lot harder than one from the six.
No, only the F4U-1A (marginally) and F4U-4 (a lot) are faster. The 1D and 1C are a little bit slower, since they have fixed pylons adding drag. I think the 1C also adds a bit of parasitic drag due to the increased weight of the cannon.
That would be neat 1200 rpm rather than 800rpm.
Yes, but as I said, I believe it was only a handful of airframes at the very end of the war that had them. If that, because I can't remember if what I read was accurate.