I would prefer fixed modifiers. Now with the relative ones an inferior weapon becomes even more inferior.
Not to mention damage does not scale well against armor. The more damage, the less affected by soak, the more one hit kills or two hit kills. TBF i would prefer just slight increase of stats over basic version.
For example, sword with 30 dmg deals 35 while triple heirloomed, this is a lot, against reasonable armor this is difference between 4-5 hits to kill to 2-3 hits to kill. I never used weapon that deal more than 32, but even it it's way more efficient at killing that something with 29 damage. Side Sword deal 38 damage heiroomed, which means even 1 hit kills are likely.
Seriously, every single point of damage counts up to 40c, as soak value is high and reduction value is fairly low. Consider the mail, with 46 armor. It substract 36,8 raw cut damage, then reduce 46% of remaining. Against weapon held by non str build (up to power strike 6) it means that weapon with damage 30 deals about 55 damage without speed bonus which means it deal about 10 hp. With speed bonus 50% it deal about 25, with 100% it deals about 40 hp. It bounces off with about -35% speed bonus, which means quite often, not to mention against plate it suck harder, even hits with 0% bounces off.
Heirloomed short sword deal 30c (+3). Heirloomed Side Sword, 36 (+4). With 36 damage you deal about 66 raw damage. I mean, in this system already power strike add percentage value, already wpf add percentage value to the damage. I would be ok with percentage increase for heirloomed items if the percentage would be smaller.