I had to re-read your post several times to understand, but thinking about it, "Yes", you probably should get the heavier WD system (1400 pounds).
I have no clue what would happen if you were slightly over the max for your bars, but assuming you were, on a smooth ride, or sitting still, it would probably have no ill effect. But if you hit that one bump in the road that would cause the trailer to porpoise (up and down), that sudden thrust of weight (bouncing), could be enough to either bend the bars or break the hitch head itself.
I tried a Google search on the affect of overloading a WD bar, but could not anything. Maybe I did't dig deep enough. But my imagination has a picture of a wire cloths hanger that is used for hanging a shirt on. Hang a winter coat on that same hanger, and it would soon bend and the coat would slip off. I can see the bars bending at the one "bump" you weren't expecting.
Considering you have a substantial investment in your truck and trailer, and you have stepped up to a heavier model now, it probably would be in your best interest to get those heavier bars. I am sure, you will never regret it.
I'm no expert, but if it were me, I'd get the heavier bars. (and hope you can achieve the same equal weight on both axles of the truck).