Or, what's the chance of the full sized off hand Rapier being allowed if I get it made of Mithral? Mithral things weight half as much, and while the books says it doesn't chance a weapons size category a Rapier is already on the edge of light/one-handed and also: racial proficiency still. So, that would be entirely changing rules, and totally your call. If it worked that way I'd be spending an extra couple hundred gold on it rather than a feat slot, or not getting to do it at all, depending on what you decide.
