Giantslayer Maths
Let's compare the impact of Lord Dominik's Regards with the current Last Whisper, shall we?
Current ADC with 1000 physical DPS.
Target has 500+ HP more than said ADC. New ADC has 1150 physical DPS.
First target: 
. Level 18, 84 Armor, +9 from runes. 2500 HP between Sightstone, R&M and Locket.
EHP against current ADC: 2500 x (1 + 0.65 x 93/100) = 4011.25 Time to kill: 4.01 seconds
EHP against new ADC: 2500 x (1 + 0.84 + 0.6 x 0.09) = 4735 Time to kill: 4.11 seconds
So, the new ADC with LDR is actually slower to kill a Janna that did not build extra armor, despite getting the full damage boost from Giant Slayer.
Second target: Janna now bought Zhonya's Hourglass to get another 45 Armor
EHP vs current ADC: 2500 x (1 + 0.65 x 138/100) = 4742.5 Time to kill: 4.74 seconds
EHP vs new ADC: 2500 x (1 + 0.84 + 0.6 x 0.54) = 5410 Time to kill: 4.70 seconds
Slightly faster with the new build. But not majorly so.
These numbers should illustrate that the new Giant Slayer mechanic should not disproportionally bolster the damage output vs squishy targets.
With these, please keep in mind that 1000 is going to be more expensive for ADCs to achieve, as their personal AD values, as well as the AD values from items AND masteries are lower than on live. On top, they are going to have less damage amplification from masteries OR are even squishier (18/0/12 or 18/12/0).
And finally, I don't think that Giant Slayer is perfectly balanced at its current values, either. It will probably see slight nerfs in one of the following patches - but to what extent is a question that must be answered by comparing live data.
