I did some quick testing by beating on a turkey with some different attacks and these were the results with +20 common combat power and no other artifacts. Reset YB between tests
20 common combat power
no artifact.......+20 common combat power...........+72 common combat power
8.................. 8.7 8.75% increase ..................... 10.7 33% increase //roundhouse
23.2................ 25.4 9.4% increase......................31.9 37.5% increase // thrust
4 ................... 4.3 6.9% increase ........................5.3 32.5% increase // punch
Turkey probably has defense stats and so messes up with exact numbers. A naked alt might work better. Also with a 50% sword the results may be a little off for thrust. It appears two points of ccp adds +1% damage.
defense test - turkey attack
with 21 common combat defense
No artifact.......+21 common combat defense....+91 common combat defense
4...................-3.6 10% decrease..................2.8 20% decrease
Pretty obvious diminishing returns, makes sense.
So it appears that a 100% sword would add only 10%, so just a bit more then +20 combat power.
Anyone else care to test out real quick and see if numbers are consistent with my test?