They are not really better than natural southpaws. In fact, when fighting southpaw, lead hand is smothered most of the time by opponent's lead hand and right leg is only good for low kicks. I'd say left-handed orthodox fighters are in much more advantageous position because they have full access to their left side strikes which are, by the way, harder to defend for their orthodox counterparts. Tumenov is a nice example of that: throws a murderous left hook and left roundhouse kick which is ideal for his orthodox opponent.