Grouse Man, I got to play with one recently...what a great LITTLE gun! Until I saw it I never would have thought my 14" barrel was long but it looks like a goose gun compared to the 8.5"
I was getting about 1250 fps with the (Federal IIRC) slugs that run about 1500fps out of a 20" barrel.
Here is some previous informal shotgun velocity test information to compare longer barrels:
Ammunition was 2 3/4" Federal Classic 1 oz. (Grey/Blue Box) Slugs.
Test Guns were 20" Mossberg 590, 16" Factory Remington 870, 14" DAC/870 Remington
Velocities:
20" : 1513 fps average
16" : 1413 fps average
14" : 1353 fps average
Velocity loss was 25-30 fps per inch of barrel lost.
More tests to come soon!
Cheers