I once suggested "optimizer".

But really, I think "rationalist" works just fine. The connection with "rationality" is immediate; as for (Cartesian) "rationalism", that's a historical term applied by academics in the specific context of an obsolete debate between (mostly) dead people that has been utterly superseded by modern concepts such as those discussed here. Does anyone visit LW and seriously come away with the impression that we're "anti-empiricist"? I didn't think so.