SoD Stitch
Banned
How bad would the extra 2 MK 103s have affected its performance though? The advantage I see is that it could've made it home in case defensive gunners disabled the forward engine (though that would make it easy prey for escorting fighters). Considering the two DB-603s could power two of the later Fw 109 Doras or Ta-152 Cs, those may have been the better application. The Do could've made a good nightfighter and fast bomber (then again the Ar 234 was still better at that). The question is how bad of a bottleneck the engines were compared to airframes and pilot availability.
I don't know how badly the additional armament would've affected the 335's performance, probably not that much; the 335 was already fairly heavy (almost 20,000 lbs. all up), so a couple of 500 lbs. cannons probably wouldn't have made much difference. Also, they had streamlined fairings over the barrels, so there probably was very little drag penalty associated with the long-barrelled (lang) Mk 103's (see picture below).
As for the engines, the Doras and the -152's used the more powerful Jumo Ju 213, not the DB 603, so there was no conflict there; in fact, IMHO, the 335 probably would've been a better a/c with Ju 213's, but they were all going to the Doras, the -152's, and the Ju 188 at the end of the War, so they weren't available for the 335.
And, yes, the -335 would've made an excellent nachtjager, especially with the heavier armament installed. There was actually one nachtjager version (A-6) completed before the end of the War, but it never left the factory.