Academy Forcemaster very much depends on whether you are a hardcore competitive player, or a someone that enjoys seeing cool stuff (Spike vs Timmy in MtG jargon). Honestly, there is very little in there that's is that good for Arena play - about 3 enchantments that are borderline inclusions, and one piece of equipment for certain builds - certainly none of the creatures. Lost Grimoire certainly adds more. That said, it does open up lots of newdirections for deckbuilding, just mostly sub-optimal ones - that matters in some circles, not in others.
Personally, I'd say Academy core is the most important for Arena in general (Crumble, Disperse and Leather Chausses alone justify it), and Tempered Faulds in Warlord are amazing. Forcemaster and Warlock are weak for Arena, and Priestess is average.