PS1 version, which is a port of the WSC i believe, has him dealing lots of damage.

If memory serves, around 80-150 around level 11, 300-600 around level 21, and 900-1500? around level 31?
But memory could be wrong.

Black belt suffers from low level games, but shines in high levels.
His multiple hits give him chances for multiple criticals, which eats through some of the more high defense monsters. His armor is very weak until extremely high levels however, and he is thus referred to as a "Mage with a cannon on his arm"

Black Belt is ok... horrible on Dawn of Souls, and not sure why you aren't seeing wonderful damage from him, he certainly outdamages a Red Mage in all my parties that have him and the RM together past level 11.

Back on topic... I have a save or two with the aformentioned party, and yes, it is a good party, but the lack of magic sort of hinders it a little, and it seems boring at times, where having more variety adds fun.
I would never really want more than 2 fighters myself after using 3 + RM. Also the RM/RW runs out of healing quite fast without frequent heal staff/helm usage, and without the additional healing of the Knights, it gets a little scarce on healing. Things hit hard even on Knight armor, and RM doesn't have.

It also depends if you play FF1 more running away from battles and skirting through the dungeon to hit the final boss, or fully exploring each dungeon, fighting every battle, and completely clearing it?

The former takes much less healing, while the latter... well.
you get the idea.

Erm just my opinion though, and by the clock it seems I'm going to be late for class at college. /wave.