While I would not really complain if the 2 20mm's were added, I don't think they should be added to the Hellcat we have now. If we ever get a quality way of providing night fighters with something to do in a way that is close to being realistic, I would be all for it. This is because, although almost all the F6F's could be armed with the 2 20's replacing the inboard 50's, I have not found, in any of my books, where this was done on the day fighters in any strength of number. Thus, I would have a hard time lobbying for it.
When/if nightfighters are added, we should definitely have the F6F-5N with the 4 50's and 2 20's.
Oh well, just my thoughts.
-math