I have yet to own a plastic/fiberglass top, but basic sound deadening technique would say that those who complain about them have done nothing to sound proof or insulate the bottom side of their trucks first. There isn't a whole lot of noise that enters from the top of a vehicle (aside from wind noise at the joints). A little bit of dynamat/luxury liner pro in the seat boxes, doors, around the firewall, and in the tub go a really long way quieting down even a soft top defender. Often times you can achieve up to a 6dB reduction in noise with relatively little effort. And that's a pretty noticeable improvement. Sealing up air leaks is probably the most effective tactic.
A fiberglass/plastic top has a very reflective surface on the inside, which doesn't do you any favors when you've got lots of noise transmitted upwards from the bottom of the truck. I would guess a headliner or any sort of absorption material on the surface would go a long way.