Blue
That is a good visual of what takes place inside the stock plenum. Adding a small wedge to the center may reduce the pressure zone and improve flow a bit. Testing will confirm this.
To add to what you have shown here remember that intake airflow is not a continuous stream but a pulse or wavelets that vary with RPM. As these wavelets stack up against the TEE they produce resonant standing waves which react a little differently than a continuous stream. In a well tuned intake these standing waves will reinforce intake airflow at desired RPMs. Changing the intake design and resonant frequency will sometimes cause these standing waves to impede intake airflow. As you said, testing is the key.