As far as the blur length, that should be only determined by the camera blur setting. All things being equal both the 2d and 3d blurs should be the same intensity, and in my renders they are clearly not.
I've used the 2d blur in the past and it has worked fine (and was very noticeable), but the focus of that animation was on a landscape, not a volumetric cloud bank. I'm wondering if the volumetrics cause issues with the 2d blur system. I'll try a couple of renders of just the landscape underneath the cloud bank to see if it works then.