Welcome to the Forum Owlfan:
Any heating system needs a free flow of air to transfer heat from the heat exchanger to the air in the house and the A/C needs even more air to do the job. The return air will not normally affect the combustion of the gas, unless it is drawing air from the room the furnace is in. That would create a negative pressure and make the gas burners go bonkers.
In my opinion the heat exchanger will overheat if there is insufficient air flow, causing it to burn out in a short time. Low flow will also put an undue strain on the blower motor causing it to use more electricity and burn out quicker. I would recommend an insulated 8" round duct picking up air from say, 2 bedrooms on the far end of the house to the return air plenum to create extra relief.