If says you may re-roll wounds, didnt said fail wounds (other weapons have "fail wounds") so this tells me yes you can re-roll all wounds to try to get the 6+
For 5pts its a good option to take if you wanted to try for mortal wounds.
Personally the 2D -1ap i think is the better option, lets say you get 3 wound hits, with a 3+ save (-1ap) making it a 4+ (50% chance) you will at least do 2 damage and a chance for 2 more damage.
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AoS: BoC 9k, CoS 3k