I recently changed my old twin-ell diverter spout tub faucet to a more modern shower head and ejector tee diverter type. I kept the twin-ell part and kept the shower head port plugged on the new faucet. I have very low water pressure now. If I take out the plug in the shower head port the pressure increases. Do I have to get rid of the twin-ell and connect the shower head to the port provided on the new faucet?

I recently changed my old twin-ell diverter spout tub faucet to a more modern shower head and ejector tee diverter type. I kept the twin-ell part and kept the shower head port plugged on the new faucet. I have very low water pressure now. If I take out the plug in the shower head port the pressure increases. Do I have to get rid of the twin-ell and connect the shower head to the port provided on the new faucet?
You do.

The twin ell you speak of is an antiquated Grohe specific part and is not designed to be used with other valves.

The newer faucets are ported smaller to restrict volume. By using the twin ell you restricted it even more.