I have tried to figure this out myself (in Nifskope) but can't even work out how to find their collision box, so I'm kinda stuck.
I was thinking maybe if collision only applied to a racers body, this might stop them getting stuck on trees, etc, quite so often (since logically they'd be able to fold their wings to glide between branches anyway) but wouldn't mean they'd suddenly start flying straight through mountains & buildings, which they obviously would if collision was removed completely.
I've just been noticing racers getting stuck in trees a lot lately, I guess....... ^_^