( Me & Nedge were discussing that in the artbook they referred to the Spirit Realm as a higher level of existence and a higher-dimension than the Universe, possibly hinting at 5-D Spirit World... But that will be analyzed another day. )
Kai in the beginning of Kung Fu Panda 3 escaped Spirit World as well, and statements by Oogway that he was a threat to the Universe seems to make him scaling to this consistent . He was stomped by Empowered Po and was stated to be far stronger using his Hero Chi alone twice in Paws of Destiny [Even Po due to calling himself Dragon Master percieves himself far stronger than he was ever before using Mastery of Chi] , meaning that Current Hero Chi Po >>>> Mastery of Chi KFP3 Po at the end of the Movie.
The Four Constellations were the only power great enough to give Jindiao the power to access the Wellspring . Thusly, the 4 Constellations themselves should be far stronger than Current Po, as Jindiao already absorbed his powers . Also, individual some of the Constellations were even able to outright overpower Jindiao w/ Current Po's Hero Chi... So they easily scale >>> Kai , KFP 3 Empowered Po , and Current Hero's Chi Po
Both Current Hero's Chi Po and the 4 Constellations were also able to harm Jindiao empowered by the Wellspring + Current Hero's Chi Po, so they shouldn't be leagues below him in power output.
Oogway was specifically stated to do the following: "With your chi I will finally be able to return to the mortal world". So Oogway should scale promptly to this as well, and obviously Kai since he has his chi along with several masters..
High 4-C, possibly Low 2-C Rating doesn't scale to: Furious Five , Ke-Pa (He was a entity Pre-KFP3 and doesn't even remotely scale to that version of Hero's Chi Po, much less the ludicrously stronger versions) , Tai Lung , Shifu , and Shen
High 4-C,possibly Low 2-C rating would scale to: Kung Fu Panda 3 Po [Post-KFP3 Hero's Chi / Mastery of Chi KFP 3 Po] , Four Constellations [Hero's Chi] , Jindiao [Post-Absorbing Po's Chi & Post-Wellspring Empowerment] , Oogway [Hero's Chi] , & Kai [Beginning of Movie/Post-Oogway's Ki Absorption]
So.... Jindiao (Empowered by Hero's Chi Po & Wellspring >=Four Constellations>>Jindiao (Post-Current Hero's Chi Po)>>Current Hero's Chi Po>>>>KFP3Mastery of Ki Po>>>Kai
Metaphorical ? It was directly stated , no strings attached , that it takes a greater power than the Spirit World to escape it . That seems as direct as it should be. Plus Jindiao being empowered by the Wellspring, the Chi of the Universe, and Po fighting him head on head should prove he scales to that high-end even without using the Spirit World.
@Numbersguy Unfortunately it seems I cannot seem to access that scan ... As finding ways to see Art of Kung Fu Panda without buying it is difficult. I do know it's specifically on page 149 of that book... I just need to find some way of accessing it. I know this because someone who had the book told me that was what it stated, and it lined up with the Wikia and what it said about the Spirit World being a alternate reality.
And the fact they should scale considering they (The Constellations) use said Wellspring as a power source, and that only something equivalent to the power of the Wellspring + Beyond can break the Spirit World according to what we saw earlier ?
Oh. Okay... For the God-Tiers... It's to be " At least High 4-C , possibly 3-A to Low 2-C " indeed. Our minds think similarly in this regard... Nothing to say there.
So the scaling is kind of like this for those upgraded : Dragon Jindiao (Empowered by Hero's Chi Po & Wellspring )>=Four Constellations (Fought Jindiao and the strongest of them managed to defeat him. Held their own against Dragon Jindiao far better than Hero's Chi Po) >>Jindiao (Post-Current Hero's Chi Po)>>Current Hero's Chi Po (Stated to be stronger than he ever was before.Fought well agaisnt Jindiao empowered by Hero's Chi Po and Wellspring , but was eventually overpowered)>>>>KFP3 Mastery of Ki Po (Shook the Spirit World in his final clash with Kai. Stomped Kai) >>>Kai (Absorbed Oogway's ki, which is directly stated in KFP3 as capable of escaping the Spirit World, which means Oogway>Spirit World. Has hi ki ontop of the ki of all china) > Oogway (Can escape the Spirit World himself, implying Spirit World <Oogway)