Good question. Microsoft is not very clear. The RDL specification for SQL Server 2008 R2 states that the expressions are "defined in Visual Basic compatible syntax" but do not contain any other details than the legacy link. Also, custom code other than .NET links for precompiled DLLs is not mentioned.
The code recorded in the report is saved as text in the RDL file after it is deployed, which means that it is interpreted on the fly or compiled by JIT when it is run in the report. For this reason, if you have a large amount or intensive code, preliminary compilation in dll and link to it in the report, this will provide better performance.