Good news #1:
I think it's easy-ish to gain 9 mm of headroom.
For some reason, they slapped a 9 mm thick "post" between the bottom of the rail and the floor where the rear bolts go:
There's plenty of space between the rail and the floor in front of the post.
It gets a little tighter at the back, but I can still squeeze my finger between the rail and the floor there:
The only explanation I can think of is that they needed a gap for the most useless thing in our cars:
the rear left floor mat.
Someone can try to shave that post off? In the worst case, slap some washers to raise the rail back a few mm.
Good news #2:
I believe I found a way to drop the factory seat further by making custom mounting brackets.
I'm starting to design them and if the project is successful I plan to share my design so anyone can order through Sendcutsend.
Stay tuned!