ETS keys: how complex in usual cases?

I've been pondering some wacky things lately regarding the keys used
for ETS tables.

It's possible to use any term as a record's key in an ETS table, but
how often is that ability *really* used?  Would it be useful to be
able to limit the type of term used for an ETS table key if it meant a
performance increase?

I'd add another option to ets:new(): {keyfmt, Term}.  "Term" would be
a representative key, examples being:

        1, foo, "foo", <<"foo">>, {foo, bar}, and {foo, 1}

To illustrate the point using the last example, {foo, 1}, all keys
inserted into the table must be a tuple of arity 2 where the first
element must be an atom and the second element must be a number.

I'm still trying to figure out if I can make my little idea is even
feasible.  If I can get it to work, the speed improvement may be 15%
to possibly 30-35% for very large ETS tables.  Would that be worth it?
Does Mnesia do anything to its ETS tables that would screw up such a
restriction?

-Scott