I'd put the Orks way, way lower than the Imperium. The Imperium actually has the tech to communicate across it's entire bulk (astropaths) and is technically a unified body under one leader (however loose that technicality is); the Orks don't and aren't. The Orks lack the ability to come together racially even if they possessed the desire, which they don't. Gazghkull managed to bring together a large number of Orks on a single planet, and the fact that that's impressive just goes to show how disorganized the Orks are. They cannot and will not unify and they don't want to.
True. I realize that when I made that post, I was thinking of how the Orks' psychic abilities work and how they are stronger when in a large group, so I was making the (incorrect) assumption that Orks would naturally want to band together - so long as they're surrounded by weaker Orks or ones that will only prove useful in the short-term. I mean, there's a reason WAAAAGH!s form at all, and it's not solely because a mega-powerful Warboss bullied millions of Orks into playing nice for a while. In this sense, it's far easier to form and mobilize a large Ork horde than, say, a Guard regiment.
As far as communication, the Imperium needs (comparatively) advanced technologies in order to remain united, but with Orks being so diversely spread throughout the galaxy (while maintaining threatening numbers in most places), they have less of a need for race-wide coordination. It's been established that if every Ork were
to somehow unite, they'd be unstoppable. As you expertly pointed out, though, they have neither the ability nor the inclination.