It looks like the “Heavy Swing” perk has either a mistake on Heavy Swing’s damage buff (it is only 3% rather than the stated 20%) or an intended cut to the damage bonus was made but the text description wasn’t updated.
Any help from warhorse with letting us know whether the perk description is mistake or the buff.xml is wrong is a big help, thanks! I think this got changed way back in the 1.3 patch but the description was never updated, and I was never sure if the severe nerf to this was a mistake, as the penalty remained at +10% stamina on attacks. Appreciate any help.
I do remember enemies dropping way too quickly in the late game in the early release, but wondered if there might be a sensible middle ground here!
Copy/paste from the game’s buff.xml (via Data\Tables.pak\Libs\Tables\rpg) below, my emphasis in bold. 
<row buff_ai_tag_id="" buff_class_id=“4” buff_desc=“buff_heavy_swing_desc” buff_exclusivity_id=“1” buff_id=“74213ddf-7a92-4a49-ae40-dca43e179197” buff_lifetime_id=“1” buff_name=“perk_heavy_swing” buff_ui_name="" buff_ui_order="" buff_ui_type_id=“4” buff_ui_visibility_id="" duration="-1" icon_id=“0” implementation=“Cpp:StrengthWeaponBuff” is_persistent=“True” **params=at1.03,wac1.1" slot_buff_ui_name="" slot_icon_id="" visual_effect="" />"