As much as I like the Do-335, it was outdated by 1945. Had it been introduced sooner, it could have made a difference, but not in those late stages of the war. Late versions of the P-47, P-51 and Tempest could already match its performance, except maybe for the armament loadout.
Now other fighters that were ready for the production line as the Horten IX were a whole different breed, but those are jets.
As a note, Dornier already had a substitute for the Do-335 planned. It's definitely one of my favourite aircraft projects, the Do P.252. Two engines driving pusher propellers, armament concentrated on the nose, 577mph top speed extracted by some quite accurate calculations. 6 30mm cannons, being 4 taters and 2 MK213C. And a 41K ceiling to catch the expected B-29 raids.