AND……How much extra range can you expect to get with a 20" bbl?

By effective range you are referring to what?
Total range:  Depends on how good of a shot you are
Fragmentation range:  See below.

Here is a section from the Ammo FAQ:

"Assuming true M193 or M855 ammo, velocity is the key.  Velocity is dependant on barrel length and environmental conditions.

As barrel length increases, the bullet is propelled faster by the expanding gasses in the barrel, imparting more velocity on the bullet, resulting in a longer range until the bullet drops below 2700 fps.  A shorter barrel has less range.

Temperature, altitude, and humidity are other factors.  As temperature or altitude increases, air becomes less dense, and bullets travel faster.  As humidity increases, air becomes denser, and bullets travel slower.

It is important, then, to keep in mind that any statistics given can only be approximate, and can be affected by a wide range of factors.  But as a baseline, these numbers are what you could expect for 75° F, 25% humidity, at sea level, from various barrel lengths:

Distance to 2700 fps  M193        M855
20” Barrel            190-200m    140-150m
16” Barrel            140-150m    90-95m
14.5” Barrel          95-100m     45-50m
11.5” Barrel          40-45m      12-15m

As you can see, barrel length and ammo selection make a major impact on fragmentation range."

There! I fixed the chart.  Looks better now.

I agree with Torf, it depends on your definition of 'Effective'.  For me it means the max distance at which the 5.56mm round can cause the fragmentation effect.  However the Army defines "effective range" as the range you are capable of hitting a point target (i.e. a Man).  For the 20" rifle it is 550 Meters (almost 600 yards), for the M4 carbine its 500 Meters.  This is using M855 (62 gr).  After that the range dial is for 'area targets' (i.e. harrasment of a squat in the open).

For the M16A1 using M193 the max effective range was 460 Meters (about 500 yards).  It is amazing to me that a mere 7 grains of bullet mass (and a reduction in velocity) increases the effective range by almost 100 meters.

I cannot define what is "effective", but I have easily hit steel plates at 600 yards with a M4 16" carbine with an Acog NSN scope.  The steel penetrators in the SS109 projectiles were fully imbedded in the steel plates.  Thats got to hurt!  Is that effective?  I don't know.  Watch-Six