The impostors are refreshed when the angle variation is larger than 1.5 degrees. If you move very quickly (at what i'd call "spaceship speeds"), you then have to update most, if not all impostors, and since each of them requires rendering thousands of particles into a texture, it can slow down quite a lot. However at "jet" speeds (i'd say, up to Mach 10), it averages 50 fps.
I still have to create a priority queue for impostors, so that at high speeds no more than 1 or 2 cloud impostors are updated at a time.
I discovered one minor issue with the "visual quality" of the impostors: for some reason, which i'm trying to understand now, the impostored clouds are displayed with a kind of white "halo" around them. It's subtle but you can notice it in these images:
The cloud in the center of the image is rendered as particles in the left screen, and as an impostor on the right one. Notice the small white halo around the cloud in the right one.