I have the overscan problem that I ‘fixed’ by commenting out the dtoverlay=vc4-fkms-v3d line in config.txt. It worked but I wondered about a performance hit and I asked about it, but I got no answer.
I recently got the fan for my Canakit case, which I did not know it supported until I got it, and now I can play games without throttling. However they were still slow and laggy and the controls did not work smooth.
I tried using the dtoverlay=vc4-fkms-v3d driver with the overscan and was able to use the menu to start games and I found they ran smoothy and fast. The answer to my question about a performance hit with the old drivers on a Pi 4 is: YES! A big hit. It will make games unplayable.
Freeciv and Flare I can play windowed to avoid the overscan problem, other games like Tux Racer do not matter much. It works a lot better than using the old drivers.
Most apps can be windowed so they are easier to use, but the Desktop cannot be windowed and that is a real problem with icons and info around the screen edges.
If anyone else with the overscan problem is using the old drivers and cant play games, I recommend you try using the new one and find other work arounds for the overscan problem.