According to the F8F Pilots Handbook for all Bearcat types, "Early model day fighters were equipped with a Mk 8 Mod 6 (gunsight), while later models are equipped with a computing sight unit Mk 8 Mod 0".
The Mod 6 was a simple fixed reticle sight that projected deflection rings and cross hairs on the windscreen. The Mod 0 was a gyroscopically controlled range-finding sight that provided lead; it projected cross hairs and ranging images on a reflector glass mounted on the gunsight itself.
I don't know that the change in the gunsight had anything to do with the change in the windscreen angle of the -2 (see http://tailspintopics.blogspot.com/2016/07/grumman-f8f-bearcat-windscreen.html) but it's possible that it was necessary to provide clearance between the Mod 0 sight and the center panel of the windscreen.
Dave Collier adds the following background: Most of the F4U-4s that I worked on had Mk 8 Mod 6 Sight Heads. Unfortunately we had two aircraft which were equipped with the Gun Sight Mk 18 Mod 6 System which used a Sight Unit Mk 8 Mod 0 that was part of the Aircraft Fire Control system (AFCS) Mk 6 Mod 0. Because this system was still classified “Top Secret” in 1954-55 we had to post an armed guard on the planes whenever the flight line was not in operation. I did not enjoy the winter nights carrying a shotgun to protect the planes. In 1956 we received AD-5Ns which had Mk 20 Mod 4 sights. He also referred me to this interesting website: http://www.aircraft-gunsights.com/usn-sights/