[Haskell-cafe] mutable arrays of tuples

So I was thinking about a mutable array of tuples, but to avoid allocating tuples to modify their fields, I guess I really want an immutable array of tuples of STRefs. Just how much less efficient is this than a plain mutable array? might it even make sense to use parallel mutable arrays? The thought of that is disgusting to me, but at least one of the arrays could likely be made unboxed...