A post a little while ago said the 7.62 round was optimized in design for a 16 inch barrel. If this is so, the RPK is not really going to get more accuracy out of a longer barrel...

So is there any real benifit to a long barrel for the 762 round?

So far with wolf ammo,my 91 is about twice as accurate as my bulgarian. I got a 1.5" group with the 91 and 2" is average for me with iron sights at 100 yds.

The sight radius is increased with the longer barrel. The larger the distance between the front and rear sight the less the error on target from slight errors in aligning the sights. That is if you align the sights on a 16" barreled rifle and the front sight post is 1/1000th of a inch lower in the notch of the rear sight the bullet will hit slightly higher on the target. If the same 1/1000th of inch error in aligning the sights is done on a rifle with a 20" barrel the bullet will also hit higher but not as much as the 16" barreled rifle would.
So the longer barrel rifle has a greater distance between the sights so sight alignment errors would be less and therefore more accurate.
Also the barrel on a RPK is larger in diameter and longer i.e. more mass, so would heat up slower. This would result in more shots before the barrel heated up enogh to effect the accuracy of the rifle.