That's a very fresh repo. I wonder how GHC would fare. I guess I should contribute.
If I were to expand it, I'd look at other allocation patterns rather than more languages; I've already good a fairly good cross-section of garbage collectors and malloc() implementations, so I don't really need any more.
So I expect some GHC specific optimizations are kicking in. Eg GHC is probably smart enough never to construct the whole tree at once. Whether that counts as the kind of static analysis static of object lifetimes someones else in this thread talked about or not, I don't know.
Update: I think it's just common subexpression elimination kicking in..