Enabled Opp Management. If a sales order is created off the opp the opp status becomes Closed-Won. Is there a way to have the opp status to Closed-Won only if an invoice is generated from the sales order?

Think about your question. What would you want the status to be if it was a sales order? It is sold and won. Your business process needs to be looked at

Good point, probably sit at "In Progress". As we want to create KPIs from opp conversions - the SO driving the opp status may not work for us as oftentimes the SOs never materialises. Invoices are usually closer to a deal happening and would be more accurate in terms of deal conversions.