the additional headroom in the sport cub, carbon cub, and xcub is largely due to the fact that the structure is laid out differently from the super cub or even our top cub. The original pa18 had a single diagonal brace; many have been modified to an x-brace, similar to what's in our top cub.
Here's a top cub with an x-brace.
Attachment 6839
and here's a carbon cub with a vee-brace; the sport cub and xcub have the same configuration. As you can see, there's no structure directly above the pilot's head.
Attachment 6840