between mana potions and +bonus damage
Warlords and Paladins...
I am talking about inherent synergy between mana potions and +bonus damage, not what some specific classes can do with them. Just about the only thing that +bonus damage affects is attacking, no spells. However, when you attack, frequently you're going to get hit back by an enemy, causing you to lose health. Thus there is inherent synergy between +base damage and health potions.
The Tri-sword needs to be decoupled from health potions. If it were attached to conversion, then it could be balanced, because every race could take advantage of it. In fact, it would power up Elves and promote new interesting strategies with them.
This would be too powerful, even if it were only +1 per conversion. For instance, an elf who converts 350 conversion points will have 5 extra points of mana and 5 extra base attack using a trisword. An orc with 360 conversion points will have 6 base attack from his racial bonus, or 9 with a trisword. It's not until around 600 conversion points that an orc decisively pulls ahead of an elf, and by this point the elf has built up a whopping 8 extra MP in addition to the attack he's getting from the trisword.
Too powerful on races with low conversion multipliers, this absolutely cannot be done.
By my calculations, the orc decisively passes the elf at only 480 conversion points, without
the Tri-sword. You say this absolutely cannot be done, yet as things are currently, the orc requires 1080(!) conversion points to pass the triling, and that's not even taking additional base damage from other potions into consideration. So it would still be a huge
improvement over the current situation.
Nevertheless, you may be right that it's imbalanced. But that's not a huge problem. If elves get better because of the Tri-sword, they don't have to stay at 70 conversion points. Alternatively, the swords can give less than 1 point of damage per conversion. There are, of course, other options.