I did read an online article on the Fairwheels website about the issues with doing synchroshift on a road Di2. Not sure that I agree but I do think that Shimano is conservative in this decision and would do it if they could stand behind it.
If I can get in the habit of calling "downshift" can I ease the big jump by simultaneously hitting the downshift (rear) button on the left and the upshift (rear) button on the right? Should I maybe do one tap on the left & 2 or 3 taps on the right? I have not changed the traditional (default) assignment of the shift buttons.