I saw Po recently got a profile so I saw it only logical to bring up the famous cosmic level feats he performs in this series. The profile already mentions one, fought Jindiao after he absorbed the "endless" power of the Wellspring which is the source of all chi in the universe, but there's more that could indicate this High 3-A stuff isn't really hyperbole. The Spirit Realm is also said to have no true boarders. It's a realm of great power, where you need an even greater power to escape, something Jindiao is stated to be able to do. This screenshot and concept art from Kung Fu Panda 3 also indicates it has at least a star inside.
With this in mind, I strongly believe everyone who scales to Jindiao, including Po, should be "At least High 4-C, likely High 3-A", if not just "High 3-A" directly.