I've gone off Seroquel but that was only about 350 mg. However I'm still tapering off Zyprexa and I was on a high dose - 25 mg (20 mg is generally considered to be the highest dose). I'm currently on 10 mg, although I've spent a couple of days on 7,5 mg but that wasn't much of a success - however, when that happened I was also concurrently tapering off an AD so that doesn't say much. I'll be going down to 7,5 mg again in 4 days.
I'm mostly worried about insomnia, I had terrible insomnia when I started Zyprexa (I took a sleep med, lay awake for a couple of hours, slept for 1-2, took a sleep med, lay awake for another couple of hours, slept for 1-2 hours) and Zyprexa cured me of that instantly. Pdoc is mostly worried about psychotic thoughts/psychosis and 'mental restlessness' (no idea what you call that in English - it's not my first language).

I hope your taper goes well.