INRIA does not support 64 bit native code on SPARC
because they do not have any such systems.  I have
a patch for version 3.09.1:

http://www.mit.edu/~jfc/ocaml-3.09.1-sparc64.tar.gz

Tk will not compile with this patch (this is a longstanding
problem that I intend to fix, but I want to fix it right
rather than accomodating the awful code generated for large
inner module initializers).

You may find 32 bit code to be adequate for your needs.

My program that uses Int32 and Int64 runs significantly
faster in 64 bit mode.  You can also expect floating point
to be faster.  Ordinary integer code is likely to be slower
because all the objects are bigger.