There is a DD cap but it's not clear how much I would say 75-85% (I said 70-75% before but I did not test it thorougly enough). Where do I base this on?
Well my murdershot hits 100-120k damage on average with 0% dd. on 70% dd upg lvl 115 (5 heroic) on average I hit 30k (no scrolls, no debuffs etc. active). Now if I murdershot dual wielding guards or dual wielding defenders who can easily get like 85-90% DD (70+ 0,3*70 = 91% DD in one layer). I still deal about 20-25k damage with murder shot, this number varies tho and one cannot be sure about how much DD people have exactly unless they show the weapons they use.