I don't believe It's the cvt ... I have driven maybe 5, 600s and everyone gives a very slight shudder when it takes off from a standstill . The dealer and service depts told me it's normal and that they all do it ?
No other hybrid I've driven does it .....

 

I think that shudder that you are feeling/talking about is the fact the 600 has gone into full hybird mode ( just the electric motor are running) and at a standstill the V8 will turn off completely and depends on how hard you press the accelerator the car will react and give you the additional power depending on your lead foot. However if you softly apply pressure from a standstill to the accelerator it will go as smoothly than a hot knife through butter (without you feeling the V8 coming on at about 30mph) with seamless shifting