I personally have had little experience with KWA rifles, but in general when a longer barrel is added to the equation is does often result in better accuracy as well as a bit more range. Just keep in mind a large portion of range is determined by the percentage of compression. The fewer air leaks a weapon has, the more likely it is to tap into it's full potential.
Simply going from a medium length to longer barrel, yes you would typically get more range. In terms of accuracy, you'll notice a more significant difference using heavier BBs. The difference in accuracy between a 375FPS weapon firing .20g BBs is pretty major when compared to the same gun firing .25g BBs. I personally didn't believe it until I mistakenly used a friends .25g BBs a few years ago.