the type of fire you are talking about is a decorative fuel effect fire (dfe). as the name suggests they are more for decoration than anything and they give off little heat as they have no heat exchanger or radiants, what heat they do give out goes straight up the chimney along with your hard earned money
with this type of fire i would not restrict the chimney in any way.
also for dfe fires the gas regulations state you have to have an air brick of 100 square cms so you are then going to get a draft blowing through your room therefore losing more heat.
i wouldn't worry too much about losing heat from the room or radiators up the chimney, is negligible.
why dont you go for a living fuel effect fire (lfe) instead, one with a metal back box which will radiate more heat into your room. valor make excellent ones, particulary the deep box versions with the slide control ignition.