You get ruffly 25fps per inch. 20-16=4
4*25=100fps

So by going from 16 to 20 inch you get ruffly 100 fps extra.

Is 100fps with it for you?

More meaningful if you rephrase the assessment by: "what is the Delta increase in distance to 800 ft-lb impact energy.  Or: to 2700 fps impact velocity?"   Suddenly the maximum terminal effectiveness window %increase can get very substantial when you compare 10.5, to 14.5, to 16, to 20

A lot depends on your environment. If I lived out in the middle of nowhere, and it was 1000 yds to my property line, I would definitely want a 20 inch AR. I would like one now, but, the max range I would shoot it at is maybe 100 yds, and it's 25 yds to my fence.

Kind of why I gravitate to a 10.5 for my choice of weapon. Does a 16 inch AR have it's place? Sure it does. Mostly because that is the legal length you can have without doing a lot of unnecessary paperwork. That's pretty much the reason for a 16 inch AR. For a shotgun it's 18 inches. At least, roughly.

The original design is a fine weapon for what it is, and performs phenomenally well with the ammo it's designed to use. A 10.5 inch barrel does great with 77 gr BTHP for a self defense round. AT 25 yds it's hits on target every time. But if I looked out and just saw open spaces, a 20 inch with a carry handle ACOG would be exactly what I would want if it came down to it.