Similar to the update to hero auras, I think the attack bonus here should be done multiplicatively rather than additively.
Currently it's +2 hack attack. Sword infantry have 5.5 attack (Mauryan sword cavalry 6.5), or 9.9 with the two blacksmith upgrades and will to fight (and even more for veteran units). I thought 20% seemed a reasonable change, equating to +1.1 without the other upgrades and +2.0 with the upgrades. (The Iberian monument was also changed from +2 to +20%.) Champion swordsmen have twice the attack, so they'll be boosted more.
Mauryan champion warriors are also swordsmen. Currently the +2 hack damage doesn't really change their effectiveness versus buildings, but +20% damage would. To keep the balance the same I reduced their base crush attack from 10.5 to 9.0. Then after steel working they'll be at 9.0 * 1.2 = 10.8, close to the original number.
These values feel right to me, but let me know what you think.