A 20% double damage chance equals 20% overal damage gain.
EDIT: The description also implies that 2nd and 3rd levels increases also the damage dealt on criticals.
Rank 1: Attacks with swords have a 10% chance of doing critical damage.
Rank 2: Attacks with swords have a 15% chance of doing critical damage.
Rank 3: Attacks with swords have a 20% chance of doing critical damage.
Where in that is it implied that each rank effects damage done instead of chance of doing "Critical" Damage?
Now, are we talking double unmodified base damage of a weapon, or modified + upgraded double damage? There is a huge difference here in the matter of damage possible, and I'd just like it to be clear on which one it is. 20% unmodified damage is 16-20 damage dealt as "Critical" damage, 20% chance every swing to deal 20 more damage? =3 seems a little pointless when every swing is automatically doing 100s of damage or more when finally maxed. Even more so when maces ignore 75% of armor increasing dps done by a tremendous amount. I haven't tried axes bleed effect, but normally any DoT is very good, even more so when fighting groups of enemies or I can imagine those dragons, that's more damage dealt while they're flapping around, which overall decreases downtime (AKA when you're just standing there shooting your bow/magic at a dragon who has uncanny dodge abilities while in the air).