Well since we can't do Player vs. Player in KB, we will never know.
Even fighting against yourself in Kordar is not the same - your copy doesn't have skills you picked up or artifacts.
That said, a warrior can still be majorly hampered by a mage. Turning those fast melee units (Vampires) into sheep and then casting mass Battle Cry on his own units, the Mage could easily achieve to go first next round, starting off with a Geyser maybe...
Or Mass Dragon Slayer, to even the chances against so many dragons.
While using a Paladin did I find that mass spells are very, very usefull in battles where the enemy is of equal strenght to you (just like in HOMM3) - they can easily turn the tide of the battle. A warrior usually doesn't have enough mana or high level Magic skills to do that.
Btw, this is what I find unfair in KB-AP: even the most measly regular level 5-8 pirate and robber heroes have like 60 or 70 Mana, and they will cast spells like Phoenix or Evil Book every round. More annoyingly they prefer to use direct damage spells to curses or blessings, and although due to their low intellect this does little damage, the insane amount of poisoning and burning will make you dread spells like fire arrow and poison skull. I once lost my green dragon to a stack of 27 Swamp snakes (and other units), their poison did 100 damage every round... I hear the patch corrects this, hope the English version already comes with this.