Sulfur poisons the catalyst (DOC) coating. It will eventually strip it off. You will not be able to maintain adequate exhaust temperatures to regenerate the DPF. Your soot output won't really change. You just won't be able to regenerate if you do strip the coating. 50 ppm, and 6 gallons of it shouldn't ruin the DOC, but it certainly will affect it. Just don't do it again and you should be just fine. If you ruin your DOC under warranty, they will replace it obviously, but it can be tested to prove that it was sulfur poisoned, and then you'll be stuck with the bill.
By the way, ULSD is a good thing for your engine. Your oil will be less acidic and you will have longer engine life.
Hope that helps.