I believe a lot of firebox doors had an inspection hole or holes to gauge the color of the fire.
Well, not quite. ONLY oil burning steam locomotive have, what you guys have been referring to as, the "inspection hole". However, the "inspection hole", or more correctly referred to as the "peep hole", is used to sand out the tubes/flues, thus ONLY oil burning steam locomotives have them. That hole is NOT used to "gauge the color of the fire", especially at night! Also, that "peep hole" has a flip-over steel cover plate, which can be quickly flipped open for sanding out the tubes/flues, and then quickly flipped closed. If that hole where simply left open all the time, the brightness of the fire would pretty much blind the cab crew at night.
Coal burning steam locomotives generally have the "butterfly" type of fire doors, but no "peep hole".
For what it's worth, that hole on the model doesn't look too big to me. However, it should have a flip-over cover plate added.
