Peformance issue when upgrade resources in SQL Managed Instance

We are doing POC for migrating customer’s current application to Azure Cloud where we are using Azure SQL Managed Instance database. We are doing performance testing for high user load to show the results to customer. We are getting degraded results if we increase hardware resources. When we use Azure SQL Managed Instance with General Purpose Premium Series 40 vCores we get good performance overall from the application and CPU utilization remains less than 50%. But when we increase vCores from 40 to 64 or more for the same test, performance gets degraded and CPU utilization remains between 95% to 100%. We have tried different MAXDOP setting like 1, 4, 8, 16, 32, 40 and 64 with different combination of Cost of Thresholds (from 5 to 500) but there is no significant difference in performance. Ideally application performance should have been improved with upgraded resource but here overall performance of the system is degraded (Execution time is doubled when vCores are upgraded from 40 to 64/80 vCores.