[Twan]

The bonus can be reserved to dwarven hammer, or the hammer of cyclops removed, this random loot never done anything good for balance anyway.
A cap can also be added (ie 50%, making a production forge bonus less powerful for units having already one).

But I see nothing bad with forge lord + production boosting forge, it would just become a logical choice if you take him, it's like ghost king or immortal + high dominion, mother of monster + high nature on her, lord of the wild + turmoil, prince of death with dominion and high death for max awe/fear combo etc... Many pretenders become better with some design choices to increase their special powers.