I can confirm that the suggestion to half the cloud size significantly reduced the fps impact of volumetric clouds on my system. Also, the stuttering is gone (p3d v2.3).
I don't like the term "volumetric" in that context, by the way. The cloud rendering is not volumetric if you adhere to the meaning this word suggests, it is just an intelligent way of arranging the 2d cloud sprites around the camera.
Cheers,
Mark