]You (must) have a much greater view distance in flight sims then in all other games. I think the main problem is, what Luthier wrote, that they have to use a trick to place so many trees in a frame. That means for me, they could not "easily" apply these hit boxes without the performance hit. No flight sim (AFAIK), except CoD, is using SpeedTree for a reason...
In DCS they tried, and then abandoned, speed tree - and they still don't have collidable trees.
In RoF they use speed tree, and the trees are collidable - but there are less of them (and less activity generally).
So...
25% less might work but 75 would be overkill.