Where/When is the use of RBAR buffering related to ASYNC_NETWORK_IO waits negotiated between the Client/SQL Server
With .NET SqlClient (and every other client library) there is no setting that forces the client to fetch and cache the entire resultset. And SSRS will read the query results as it renders reports, and whether it reads the entire resultset at once will depend on the report design and the rendering destination.

You may be able to get SSRS to read the results more quickly if you use Shared Dataset Caching.

But it's not obvious that there's a problem that requires fixing.  You have a report that consumes 800K rows.  Forcing it to fetch all that data at the beginning is going to be expensive on the report server, and may not make anything better on your SQL Server.