Frankly, all that matters is Imu is potentially stronger than everyone, not necessarily the Gorosei. They don't necessarily need to be above BB/Shanks/Akainu to be endgame, because they are subordinates, though its possible some of them are depending on where Zoro, etc are at that point.
Imu's subordinate count is also ridiculous compared to the rest:
- Akainu presumably has 9 major subordinates (2 Admirals and 7 Zodiac VAs, discounting Kizaru if he's gone this arc)
- Blackbeard has 10 major subordinates (if you count Kuzan)
- Shanks has 9 major subordinates
- Luffy has 9 major subordinates
Imu has, on paper, at least 14! 9 God Knights that we know of and 5 Gorosei, all of which "seem" to be relative to an Admiral potentially. So unless they lose this arc, Imu is not balanced like the rest are as a major group.
Random tangent, but Doflamingo may have categorically similar with his 4 suit Executives and those under them. I mean, compare these scenes even!