How to roll a fast BVH representation in Haskell
If I understood you correctly you want unboxed arrays of user-defined types? if so check-out the vector package which also supports loop fusion. It's worth checking out slides for High-Performance Haskell
I should really point out that Haskell is not very good at giving the programmer a means of choosing data layout in memory.
You might be interested in storing the tree in a flat array in cache-oblivious way ("Van Emde Boas tree"). It should work, but who knows. :)
(shameless plug: I've made a similar effort some time ago; I've used some advanced type system features of the ATS programming language to make the raytracer both safer and faster; see the code here: http://code.google.com/p/ats-miscellanea/ -- I didn't go very far yet, unfortunately)
What you're proposing was discovered years ago, it's called a bounding interval hierarchy (BIH).