This 7DTD mod adds some quality bonuses to both Entity and Block Damage.
It is percentage based for now and adds 20% per quality level to entity damage (0% for 1, and 100% for level 6). It also adds 40% to block damage per quality level (0-200%). Luc added this in, as it felt like there was less of a reason to worry about quality level unless you had mods on hand, which are few and far between.
It should stack as a modlet without issues, it removes all other perc_adds to items and then adds these to all items found (with special changes to items like the AK that normally have custom perc_adds).
The forum topic of the mod is here.