Not sure what model you have but if I did that, I would be covering the slotted heat shield that allows for searing. I have the Classic model. I use that searing feature on occasion.
I have the slotted head deflector, with the additional piece on top. I foil both of them after every cook. Even so, there is some cleaning to do underneath, as that is where the fire / soot end up.
Tonight we did hamburgers, and I used the pit boss for searing. After 1 hour in hickory smoke, I used a kitchen knife to cut the foil in above the slots, turned it way up, and got the flame going pretty good.