A quick check in QRG tells me that my 78 Wizzy with UC3 and Mana Neck XI hits for 5.7 mil and crits for 10.5 mil using Yamakagi's IV.
When I remove my UC3, the damage drops down to 3.7 mil and 4.5 mil respectively.
Did this in about 5 minutes so, take those results for the small data pool that they represent.
could i trouble you to try just taking off your mana neck - if your dmg does down then that would tell me everything i needed to know. If your dmg goes down by a couple mil then that would tell me they do stack.