I vote yes, it will be too heavy to be a good flying r/c model. The rubber kit is built with stringers and covered in tissue which is very light. A molded fiberglass model is a solid sheeted unit (no gaps between stringers where only the tissue is
). If you used even extremely light cloth and a structural laminating resin (MGS, Aeropoxy or similar
) I think by the time you got it structurally sound, it would weigh double the intended weight. I could be wrong, but if you want to mold a Hellcat, I would go with the 60" span idea, since the amount of hours will be roughly the same to mold either one. With a larger model, the wing loading will decrease as the area goes up (30"x 10" = 300" sq., if you double that, 60"x20" = 1,200" sq.
). You will also have an easier task adding more scale details to the model with it being larger.
I don't want you to give up by any means, as molding an airplane is very rewarding in you knowledge and self accomplishments.
I just think you would be happier with a model that looks good and fly well too.