I just used a 20" paella pan to make a large paella for 10 people.  The pan covers 4 burners.  The problem that I had was that the center of the pan never reached the temperature of the parts of the pan under the burners. If I raise the gas levels enough for the center to get to a boil, the outside areas get too hot and burn.  Difficult to make an even paella.  Any suggestions?  I have looked for, but not found any heat diffusers that are large enough for this pan. 
The method I use is to start the paella pan on the middle of the biggest burner, then once the rice and liquid is in I move the pan over the burner every 2 minutes to make sure it cooks evenly. For example I'd move the pan so the left side is over the burner first, then move it to the right, then top and then bottom, keeping with that pattern.  