They don´t underperform that hard. The groupbuffs from dancer and bard supports a group very well. Maybe the machinist could need a little buff to compensate this support-abilities. Overall they´re fine and you can play any dungeon and boss easily, meanwhile caster and melees have to adjust way harder especially with such RNG mechanics.
That they´ve 1-2k dps less overall seems pretty fair to me in this way. It´s not even really possible to balance it out. If we take monk and blackmage as hardest examples, it´s always boss-mechanics relative to be in a comfort zone or not. Physical range are always in a comfort zone. If you´re going to bring the phys. ranges on the level of them and you hit a boss where a blackmage have to move 24/7, then it´s not going to work and a blackmage will underperform heavily with a minus of 3-5k dps probably.
You can´t take the top dps as example anyway. You´ve to look for the average player. Not every melee is able to play around the serverticks, to hit every off gcd on a perfect cooldown etc. Same as caster can be interrupt by any mechanic aka E11s tether-bubble. And this is just pure random. Average or even above average players will do mistakes here and there and won´t find the perfect rotation for any moment. This have to be calculated to keep this game fun.
The next thing about melees are uptime strategies. Of course it´s good to have them, but i don´t think that SE always "thinks so far" or just ignore such possibilites when they design a boss. This makes a huge difference too, when they calculate the dps based on their own expectations "how to beat the boss".
That said, if you bring phys. range on the level of the others, they´ll outperform them any time and the most players will lay down on it, because it´s going to be way easier to play mechanics without any damage loss. If anything, then SE should take more care about boss mechanics, possible uptime strats, etc. E.g. A boss with less movement mechanics should have higher magical resistance to counter the power of a blackmage in some way.
Instead of buffing even more classes, SE should nerf the top DPS. That really good players are able to skip 3-4mins of the actual content is a joke in my eyes. Blackmage and Samurai are too strong for a long time now anyway. But phys. range? I´m convinced that they´re in a good state and bring a lot to any raid-party.