Undersized Return Grille Question

I have a two story house with 2,170 sq.ft. and their is only one return grille that measures 18 x 24 in. The return grille is located on the 2nd floor right in front of the stairs. The A/C unit is a 3.5 Ton heat pump. I read online that the Return needs to be 200 sq.ft. per ton. Meaning that my return should be at least 700 sq.ft. and right now is 432 sq.ft., which is half the size its supposed to be. Its actually smaller because its not an open space, but a funnel type of opening. I was thinking of ripping everything inside and covering the inside walls to allow more airflow. Also upgrading the return to 20 x 30. I was even thinking of changing the return grille to accommodate the thicker 4" filters that are less restrictive and last longer.

Can someone tell me that is that device that is attached to the return?

Pictures of Return:
https://imgur.com/a/pIm9JaH

source https://hvac-talk.com/vbb/showthread.php?2222439-Undersized-Return-Grille-Question&goto=newpost