Also one can increase quality and quantity of circuits even in magic starved nesuverse. Still they can maintain quality. Look at royal blue blood circuits of barthmellori. Also with Age of God in effect I suspect quality of circuit will not decrease rather increase.
Can I get a source for that? Because everything I have read or heard about magic circuits that isn't fanon says that the quality and quantity of natural magic circuits can't be changed, there is artificial ways to add on to the circuits you can use like a Family Mage Crest grants the possessor access to the magic circuits in it and you can make more via your own nerves but it is painful, inefficient and as far as I can tell, temporary. Mordred can improve the quality of her magic core(the big brother of the magic circuits) because of CYOA fuckery and becoming a goddess gives her literally divine magic which is better in quality and quantity.
Unless you are talking about how over the generations Fate Mages can get better and more circuits? In the comment I said they will be static I was referring to how any ONE person can't increase the quality and quantity of their natural circuits, well, more like magic cores since they will be part dragon. The magic cores they are born with will be different per person much like the average magi in fate.
An impact on what? Their fate magic? Yes. Their warhammer magic? Yes, it will impact them much like how Mordred's affinity with Lightning grants her aid with casting LIGHTNING spells, not Azyr spells but lightning spells.
One of the biggest problems is that Mordred is mostly self taught in magic with a few scraps from her mother tossed in. So, even if she wanted to do stuff with fate magic, she would be starting from stratch with only the knowledge you would expect pre grail war shiro to have. The only advantage she would have over him is the having her magical blast and the years spent trying to surpass it.