The fireplace produces radiant (short wave) and convective (long wave) heat. By far, the more useful is the convective heat that passes through the shell and is circulated to the room. The glass doors do prevent some radiant heat from entering the room. Radiant heat feels warm but does little to actually increase the air temperature. Operating the fireplace with the doors and screen open may increase radiant output, but its unsafe, and allows lots of heated air to be drawn up the flue. This can result in a net loss of heat in your home as makeup air infiltrates from outside.
The majority of useful heat in a zero clearance fireplace comes from air circulating between the shells, and returned to the room.
I don't recommend installing a stainless panel over the refractory brick (I think thats what you mean), because, it will become non-reflective after a short period of use, and its a modification not intended by the manufacturer. Again, mainly a radiant heat enhancement.
You are installing a 20 year old fireplace. I can understand this seems economical, but if you are really looking for heat recovery and efficiency, the built in wood stoves are far warmer, cleaner and safer.