I think the KPIs should be designed to measure what you set your synergies (i.e., reason for the M&A in the first place) to be at the beginning. So if the synergy was to increase market-share by merging, then the KPI has to measure XX% increase from baseline in market share. If the synergy target was to expand into a new therapeutic area, then the KPI could be comparing the number of key accounts the target company had originally to the combined companies 6 or 12 months later and see how much it has expanded. If the original synergy target was to lessen competition, then the KPI could be measurement of the economies of scale because of the merged organization.
Most of the posts were measuring retention of employees, but that may not be useful if they are paid retention bonuses (it is almost a false sense of security they are staying for the money, not because they love the combined company). My experience in measuring KPIs is that the measure really should be hard with actual data – not sure how you measure successful communication or quality as soft measures – they would really need hard data to be an accurate representation of the success.