*cough* terrace farms *cough*I don't think it's wise to give Egypt or anyone a strong one per city improvement if it comes early like the sphinx. Maybe if it got progressively better as you advance through the civics and technology tree. An early hockey arena or open air museum would be broken when you only work a few tiles per city.
The most broken UI in the game which you get at the very start. If the devs consider this balanced, I'd say adding a further 1 food and 1 production to the sphinx shouldn't make it that imbalanced.