IMO Plox is all about metagame-dependant techs. I'm running a Spiritomb/Upper based Plox like you. However my has a little more techs than yours. I like Mewtwo LVX even though people says it ain't that good anymore. It gives you some extra time against ShuppetDonks if they have forgot their Regice and SP variants missing Dialga is auto-lose against Mewtwo. However I found Gallade pretty useful against tanking Dialgas though. The first one can be OHKO by a full-prize flip if necessary. Losing a fully tanked Dialga in one shot is devastating for such a deck. If they recover some how you can 2HKO with Gallade's both attacks.
Also I find it rather unnecessary to have techs specially for mirrors. Lake Boundrary will probably back-fire on you in mirrors. Breloom + Moonlight Stadium, is a more safe OHKO of Gardevoir's. On top of that it is an excellent counter for a lot of decks based around free-retreat pogies like Flygon, Garchomp SP, Luxray SP, Gengar (Fainting Spell is a bish though) and things that get affected by Moonlight.
Dusknoir which you play is universally useful so I wouldn't exclude. Overall I still find the GG competible, but it's rather sluggish being dependant on Upper. However with DCE coming that problem will soon be solved.