Overdraw was a challenge, but keep in mind that there's not a whole lot else going on in the level. Well, modelwise. No terrain, only a flat plane with London painted on it. It's just clouds, and loads of gameplaystuff. When you fly through the clouds, they do indeed cover the entire screen.
If I remember correctly, on Ps4 and Xbone, the clouds used something like 1500 particles for the entire level.
Another thing to keep in mind is that the level of "badness" overdraw causes is also based on the complexity of the shader. We didn't use any custom shaders. These are simply lit, non-normalmapped sprites. Nothing fancy going on that's going to make them more expensive.