To quantify the overhead cost of TDE, one approach is to run a test workload, something like replaying a profiler trace, against both a TDE version of your database and then against a non-TDE version of your database. To make this more manageable, perhaps create a database called [TestNonTDE], and copy in a manageable subset of tables (with indexes and statistics) that will cover your test workload. This database will remain un-encrypted. Next, backup this test database, restore under the name [TestTDE], and then setup TDE on that database. So you're comparing baked apples to raw apples, you'll want to host the test databases on the same instance and storage (not necessarily production), and also run the workload test system off-hours when there is no other activity. Also consider re-starting MSSQL service before each test workload to clear the buffers and reset DMV stats.