I agree with Will32Rod
that a 20" diameter pit can be less amenable to tuning plates than a 24" diameter pit.
The most immediate issue would be that the tuning plates would most likely be fairly close below your bottom meat grate, and contribute radiant heat to the bottom of the meat, so the bottom of the meat might be exposed to more heat and require more flipping and management.
Even on a 24" diameter pit, there can be radiant heat concerns, depending on how high the plates were installed.
has a 24" diameter pit, but had issues with radiant heat (due to the tuning plates being installed 3" below the bottom meat rack):
Which he addressed with this modification:
To me, tuning plates are best matched when you are cooking a large volume of similar meats and basically filling up your pit. Of course, if you're mixing different meats, sometimes it really is more advantageous to put meats that prefer a hotter temp closer to the firebox on a pit without any tuning plates. It can also take some experimenting to get tuning plates right, and particularly because from what I understand, pits "tune" differently when they're empty, partially full, or full of meats. So it would seem to me that they would be best when a person is cooking the same consistent batch of meats (quantity and type) over and over again - i.e., more like a caterer or other large diameter pit used for big gatherings.