Note that you’ll get more packs and rares with the end of the month rewards if you climb high enough.
So even with 15 it might be enough if you add those extras in. Unless you’re still missing a lot of other rares from previous sets.

“Note that buying the full mini-set bundle for 2000 Gold is cheaper than buying 25 packs!”

That’s a fair point, but I already have 29 Darkmoon packs hoarded and ready to be opened.
With that I’m basically guaranteed to get all the cards except the Legendaries.
If I do buy the miniset with gold, all the packs will have nothing but duplicates which kinda feels like a waste.
So it all comes down to how good these Legendaries will be, and how many decks they will go into.
I know it’s impossible to exactly predict how much play they might see, but honestly? Just from looking at them, they all seem kinda shitty.
I’ll wait a week to see, but I’m almost certain I’m not buying it with gold just for those legendaries.
The only other benefit I can see is the access to Duels treasures and hero powers, but I don’t think that’s a good enough reason unless you enjoy playing it.

So here are the reviews for Scholomance:
https://www.hearthstonetopdecks.com/scholomance-academy-card-review-1-devolving-missiles-lightning-bloom-rattlegore-shando-wildwalker-troublemaker-and-more/

And some of the ratings:
Devolving Missiles Card Rating: 6/10
Diligent Notetaker Card Rating: 8/10
Shan’do Wildclaw Card Rating: 8/10
Wand Thief Card Rating: 6/10
Animated Broomstick Card Rating: 5/10
Natural Selection Card Rating: 4/10
Headmaster Kel’Thuzad Card Rating: 8/10
Steeldancer Card Rating: 4/10
Disciplinarian Gandling Card Rating: 9/10
Cabal Acolyte Card Rating: 6/10
Ace Hunter Kreen Card Rating: 9/10
Glide Card Rating: 8/10 (to be fair everyone got this one wrong)
Felosophy Card Rating: 8/10
Pen Flinger Card Rating: 3/10
Groundskeeper Card Rating: 9/10
Adorable Infestation Card Rating: 6/10
Runic Carvings Card Rating: 8/10
Void Drinker Card Rating: 8/10
Shadowlight Scholar Card Rating: 9/10
Instructor Fireheart Card Rating: 8/10
High Abbess Alura Card Rating: 6/10
Forest Warden Omu Card Rating: 9/10
Primordial Studies Card Rating: 6/10
Wave of Apathy Card Rating: 6/10
Overwhelm Card Rating: 8/10
Reaper’s Scythe Card Rating: 8/10

Pretty funny stuff overall.

“Zephrys the Great […] However, it requires a Highlander deck”

No it doesn’t. Murloc Paladin and Quest Druid would like to disagree.