Technically, it would be possible. However, there are some major disadvantages in doing so. At the moment it doesn't matter how many slides there are in the slideshow, it only has to calculate move two. This greatly improves the slideshow's performance. If the slideshow would need to slide back to the beginning, it would have to calculate all slides in the set amount of animation time. This would cause a lot of lag, even more when your website is responsive.
From a programmer's perspective, it would be ugly to implement. When doing an animation, the slideshow only knows about the two slides that are animating. If the slideshow would have to scroll back to the beginning, all slides would have to be looked up and put in line for animation.
Did you like the 'whizz backwards' animation because it felt more natural, or are there any other advantages in doing the animation like that?