Ok, here we go. Basically this is just a kill % list that I made trying to prove to a stubborn DK main who said that Lucario can't kill until well over 100%.
For the testing, I just used fsmash for a few reasons (common kill move, easy to constantly sweet spot without needing to control DK etc)
Lucario % DK %
0%
Uncharged 180
Charged 125
50%
Uncharged 145
Charged 95
100%
Uncharged 110
Charged 70
150%
Uncharged 90
Charged 50
200%
Uncharged 80
Charged 40
Now, as we can see, the longest DK is ever going to live will be until 180%...assuming Lucario has 0% and they are both in the middle of final destination. The earliest he will be dying will be at 40%, assuming lucario is at 200% and again in the middle. Both are major extremes that will probably never happen. Looking more realistically, when lucario is at 100% or even at 75(didn't do a reading but estimating) Dk will be dying at around 120ish%, which is extremely possible. Lucario definitely doesn't need to be above 100% in order to kill DK early.
Now on the other side, for DK's killing %'s, I used fsmash, due to it's killing power.
Uncharged 85
Charged 50
A fun fact, Lucario's fsmash has quite a bit more range on it...
Now, it seems that lucario has to be about 85% in order for DK to kill him with an uncharged fsmsash. at that point, DK only needs to be at about 115 in order to be killed by lucario. To me, that doesn't seem like DK has an advantage as big as you were making it out to be, in the area of killing power.
Add lucario's ability to gimp DK, which is much greater than DK's ability to gimp lucario and the matchup is definitely even, or even going in the favor of Lucario.
Just an fyi, all damage %'s were done in increments of 5 with the number shown being the earliest the character died ie if it says 50%, the character didn't die at 45. Thinking like that, any number between 46 and 50 could of been the % that the character actually died at but with all the lucario %'s , I didn't care enough to test it.
Also, while fooling around, I found a few moves that go through DK's upB (or at the least cancel it and both characters take damage/knockback):
fair
dair
bair
nair
uair
yeah, all of lucario's aerials, with dair, bair and uair completely hitting DK without any damage to lucario.
Personally I think it's even but with a slight advantage towards lucario (not sure whether it's enough to justify an advantage.). He can kill DK relatively early and if you add gimps he's going to be getting a lot more kills than DK will.