I have used both, but in road racing, not in sprints. I was road racing as the bodywork transitioned from what was common in the early 90s at the club level, to full fiberglass. That full bodywork is being mimicked by the Gold Cup as so far as the general shape of the nose. With the sprint plastic, terminal velocity and the speed at which it’s reached were both noticeably negatively effected. Sure, it won’t have AS strong an influence in shorter tracks with their smaller top speeds and taller gear ratios, but we all know that the effect of a draft starts even at slower speeds.
So what this means in the current debate is that, when the “full” plastic is pitted against CIK-type plastic, the full plastic will have a speed advantage. As 4 cycle racing continues to return in popularity in sprints, someone has to make the unpopular decision, and soon than later, because, regardless of the decision, a whole lot of racers will be upset. Waiting will only make the number of upset karters larger.