Then you couple this with an autopilot that in IAS hold can give a ROD of between 1000 FPM and > 4000 FPM
Yes, it really is that bad. Use of IAS above FL100 is not very sensible in my humble opinion, particularly in the descent. Not difficult at all to descend in VS and accurately keep the desired speed using the throttles. I personally favour VS for any descent except if you intend to descend at idle, which is quite rare. Accurate control of the vertical profile using IAS and power is nigh on impossible.