T'would be great but it would make no sense unless some serious revamping in night combat would take place like a different radar system and much lower icon visibility range.
Therefore there's really no point for a night-fighter Hellcat and no need for the 20mm armament. This game's about what entered combat in the most common/standard configuration. No 20mm on Hellcats. Please use the search button
Lastly, it's not F6-F5. It's F6F-5. It bothers me to see the former published on websites all over the place. Not even Wikipedia makes that mistake. But don't worry, it's not bad for you it's just annoying when published sources have it.
F = The Naval term for 'Fighter'
6 = Sixth of this type (in this case, fighter) to be built by the company
F = The Naval code for the Hellcat's company; Grumman.
5 = The fifth variant of the F6F; a figure represented and separated by a hyphen.
Similary, you'll encounter other Naval codes. For types, we have SB = scout bomber, PB = Patrol Boat, TB = Torpedo bomber and the list goes on.
Company codes include: A = Brewster, C= Curtiss, D = Douglas, F= Grumman, G= Goodyear, H= Hall, U= Vought etc.