In Toad 11.5 you can set the Plsql_optimize_level parameter value from Toad Options, Compiling, Set optimizer compiling value. However, the highest level is 2, even if Oracle 11 added a new value 3.
If you want to ensure compiling at a high level on Oracle 11, do it in another way. Perhaps this should be fixed at some point.
Best regards Niels Jespersen