Just to clarify: the new grt 10.1 efis units do not have a built in ARINC port (older HXr and Hx efis units do have arinc ports), so you need their external arinc box. The external arinc box communicates with the efis by using an rs232 port. But, the arinc box also includes an rs232 port, so you don’t lose any. However, the rs232 port on the arinc adaptor is limited to 19k baud rate.
As always, it’s important to see if you have enough rs232 ports, considering baud rates needed, etc., for all your needs.