Don't slash anything with power up punch, it is an absolute necessity. SD that goes through crap like tyranitar and balloontran with sucker punch is sort of half the reason to use the pokemon.
Slashing sub was specifically for defensive ghosts types, admittedly this isn't so relevant at this point because of the lack of Jellicent but having sub is actually a huge help against pokes such as Cofagrigus, Trevenant and Gourgeist who can easily switch in and will-o. Once you have a sub up against these mons you're pretty much golden. Also, as i suggested in the op, if you're running sub, you should also be running drain punch so you can hit the threats you mentioned. It's pretty much there in the instance that your team doesn't have a reliable way of dealing with ghosts.