Well...
Dreadnought charging...you probably want to see a flamer hit in there, so 1(to hit) * 1/2(to wound) * 2/3(bypass cybork) = 1/3 wounds on the warboss.
Dreadnought attacking with 3 attacks at I4 --> 1/2(to hit) * 5/6(to wound) = 3*5/12 = 1,25 wounds on average that will insta-gib the warboss. He can use his cybork save, reducing this to 0,833 wounds.
If the Warboss is still alive after this due to nice cybork saves or bad luck for the Dread:
4 attacks, hitting on 3+, penetrating on 3+...1,778 penetrating hits. 0,593 of these pen-hits will wreck the Dread on average.
If the warboss charges:
Dread has only 2 attacks, so 0,833 wounds on average that insta-kill the boss, reduced by cybork to 0,556 wounds on average.
Boss hits at I1, if hes alive...2,222 pen hits, 0,741 destroying results on average.
When Dread charges through cover, he strikes at I1, too, with similar chance to kill the warboss. The boss gets his strikes, too, though, so with a bit of luck both will die - the Dread has still the better chances I think.
For without cybork, consult calculations above.
This is quick and dirty mathhammer that gives not perfectly accurate numbers, but only rough averages of what will happen. If I did something wrong, someone correct me please.