Something tells me the Virtus is to the MCX what the Legion is to the P-Series. Basically, you're getting a much better trigger, heavier barrel, M-LOk rails, and some color options for about $300-500 more.
Perhaps the heavier barrel profile will address the accuracy issues that some 16" 5.56 rifles had after strings of fire.
As for full ambi, that is a bit of a stretch since the bolt release is still left sided only. Why they don't make a full ambi lower like the IWS or their own MPX is a bit mystifying.