I was in the Training Hall and I decided to do a comparison of damage done by various swords. Here are the results.
Shield Aegis (5*) Barbarous (5* with Medium Sword Damage)
Sword proto (0*) 15 16
Calibur (2*) 30 34
Ascended Calibur (4*) 36 40
Cold Iron Vanquisher (5*) 33 37
Leviathan (5*) 39 43
Combuster (5* brandish) 39 43
Divine Avenger 43 47
So we can see that medium sword damage means a 10% increase, Barbarous being the only shield that offers this. (Few shields have any abilities)
Each additional star adds 5% damage ability to a sword. I also tested a 4* shield and saw no drop in damage ability. A 2* shield, however, results in a 5% decrease in sword damage.
Nicely done