I noticed recently when playing EoD (which is heavily based on WiF, I think) that pocketed units do not lose organisation; they lose strength. This was a fairly unpleasant surprise since they can still fight fairly effective. If that's the case in your game, too, reducing the 150 division pocket (if the operation succeeds) may take a fairly long time. I remember that I had to let Moscow starve for half a year to have a chance to take it, and in my game there were far fewer divisions there.
True. They do regain ORG extremely slowly though, but they do maintain it. It is better than HOI2DD's 'get encircled and get forked' strategy. I've had many lucky breakouts from enemy encirclements in my day, both by my pincer attacks failing or by stupid AI retreat moves out of my control. In my old AAR i remember it took me about 3 and a half months to massacre 300 or so soviet divisions (and they fixed the mod cuz of me )