I believe that PMI manager should have both financial and non-financial KPIs and these KPIs should correlate with the key acquired company metrics.
Financial KPIs: Revenue Growth, EBITDA Margins, Cash Flow Performance, Return on Investment.
Integration KPIs: Integration Milestones deadlines, Synergies Realized,
Cultural and People KPIs Employee Retention Rates, Cultural Integration Metrics (feedback, surveys results), Employee Productivity/.
Customer and Market KPIs: Customer Retention Rates, Market Share Growth, Net Promoter Score (NPS), Product or Service Adoption Rates.
Using a balanced set of KPIs should provide a comprehensive view of the integration’s success and identify areas requiring corrective action.