Probably the reason why your kabar bites better at the serrations is because the geometry there is thinner than the rest of the edge.
Given the same thickness I just don't see how a serrated edge would chop better than a plain edge. The serrated edge actually has more cutting surface to bind and give resistance during a chop. They also cut at different angle in relation to the straight edge portion. Slicing and shearing motions on softer material may be a different story depending on how the serrations are done. A stock AK already has pretty thick geometry at the edge. Putting scallops that ride higher up on the edge will only make the edge thicker. Now I can be entirely wrong here....but if I had to bet I would put my money on a plain edge for chopping performance.