Hi and welcome to simply discus Hynsicker .
You can never have over filtration and the general rule that applies is the more the better .
However there is a point when adding more biofiltration is becoming usseles because benificial bacteria is in an equilibrium with the ammount of ammonia produced by the fish . Since we can't track that amount easily and can not quantify the beneficial bacteria its always safer to have more biofiltration to ensure this equilibrium .
With that said , there is a lot more going on in the water than just nitrates and ammonia cycle and that's why over filtration can not guaranty you clean water if you don't change it with new fresh water. Water changes are extremely important for discus , regardless of the filtration included.
Another thing I want to point out is your tank volume. Its too small for keeping anything else than just a proven pair of discus in there.
Discus are big and messy fish that require pristine water conditions and lots of space , that's why a general stocking rule for them is one discus per 10 gallons of water .