I don't think the Cabal synergy is quite enough. Shrinkmeister had some extra utility because it was attached to a body. Usually with combo cards you need the effect to combo with several different cards in your deck.
This is what I meant when I said that either the synergy was poor (madness effects, shadow word pain) or the cards it synergizes with is individually poor (shadow word horror). Cabal is the only good option here. It's sort of like how Cho'Gall is only super good with Siphon Soul. That wasn't enough for Cho'Gall to see play.
The thing with pint sized potion is you lose the body but get way more out of it since you gain the effect board-wide (and it's a more POWERFUL effect) along with a mana discount which I think is an easy trade. The 3/2 was never a body you actually wanted in Priest since those minions need a power word shield for the hero power to do anything. The dream combo everyone would want with Shrinkmeister back in the day was grabbing Ysera, opening this up to 5 attack is a big deal if cards like Syl, Emperor, Thing From Below, etc. are all added to the mix on top of 4 attack minions. If shrinkmeister himself already saw play I would be surprised if what is just a stronger version doesn't, even if you lose out on the option of playing a 3/2 on 2(which, again, is shit for Priest anyway). I sorta feel the same about that Druid card that's just 1 mana +1/+1 to all since they don't want to just play a 3/2 instead, only this is more powerful over the base card.
Horror is a bad card but control Priest always has situational reactive cards and that combo seems too good to not work out. Like every single minion midrange shaman plays except fire ele just die to it, any minion attack range on the field 0-5 all just dying atleast needs to be heavily tested out.
Cho'gall spells that a renolock would play are usually board clear centric which works out poorly. Outside of their pool of 1 mana spells they're almost all like this -- hellfire is bad since it basically kills you, deals 7 damage and makes cho a 7/4. Shadowflame has potential but the card typically isn't hard to trigger and you want it to be a swing where you pull ahead, paying cho'gall with it is pretty slow. Twisting nether is obviously bad since you pull it off 1 turn earlier which dealing 8 to yourself. Siphon ends up standing out compared to all of those but is single target removal. You can basically look at Cabal as the single target removal and horror as the powerful sweep.
The shadow madness card is more of a question mark for me. The card is always best when the meta has sticky garbage so you keep something out of it or deny something strong. Obviously had a ton of targets pre-standard with scientist, creeper, nerubian egg, knife juggler, etc. Right now this is only looking like Hunter so something like Kindly Grandmother woudl be good but that's about it. Bad Rat is also good now but if the grimy goon stuff works out the buffs might actually pull it out of range easily.